In June, Sen. Mark Kelly of Arizona was the only Senate Democrat to stand alongside President Biden as he announced a new executive order aimed at cracking down on asylum seekers.

“I’m happy to be here today, and what I hope is that we will have a safer situation and operational control over our southern border,” Kelly told reporters outside the White House that day.

Kelly’s eagerness to associate himself with a tough border measure just weeks ago is now suddenly seen as his greatest asset as he viesto become Kamala Harris’s running mate in a transformed presidential race against Donald Trump following President Biden’s sudden exit from the contest.The two-term senator and former NASA astronaut is fluent in and brings some credibility to what’s perceived as Harris’s weakest issue as a candidate: security of America’s southern border.

“When it comes down to it, [Kelly] has a record that can really protect against a withering, brutal assault on the border on her that’s going to be coming,” said political operative Mike Madrid, who worked on GOP campaigns in California and later co-founded the anti-Trump Lincoln Project. “You can’t lose by 35 points on the border to Trump and win Pennsylvania, Wisconsin or Michigan.”

Trump is already going on offense against Harris on immigration — his first ad targeting the likely Democratic nominee focused on her record on immigration and called her “dangerously liberal.” Harris has received an onslaught of criticism for her role, assigned by Biden, seeking to combat the root causes of migration to the United States from Central America,with Republicans calling her Biden’s “border czar.”Illegal border crossings have soared during Biden’s presidency, though they dropped sharply in recent weeks because of the shift in administration policy.

Some hope that Kelly, who declined an interview request through a spokesman, can help inoculate Harris on an issue that Trump is certain to make a linchpin of his reelection campaign.

Amanda Renteria, a top aide to Hillary Clinton’s 2016 presidential campaign, said choosing Kelly would “broaden” the Democratic ticket by diversifying the experience on it. Kelly is a former U.S. Navy pilot who flew combat missions in the Gulf War, and he would represent a stylistic departure from Harris. “It shows her choice to bring on people that are different from her,” Renteria said. “It’s a huge contrast to Donald Trump and JD Vance.”

While many of Kelly’s Democratic colleagues railed against Trump’s desire to build a wall on the southern border and criticized him from the left on immigration during his presidency, Kelly has sounded an entirely different note. He pushed the Biden administration to help close gaps in the border wall in the Yuma, Ariz., sector in 2021, and in 2022 criticized Biden’s decision to lift a pandemic-era policy allowing officials to turn away asylum seekers as “dumb.”

Kelly has not spearheaded meaningful legislation on immigration during his short time in the Senate, but he was a vocal proponent of the bipartisan border deal that ultimately failed in February amid Republican opposition. He has built deep relationships with border officials, law enforcement and migrant rights activists in Arizona, giving him a rare fluency on the issue, according to observers. Even some officials who criticize Harris for what they see as a lack of concern about border communities praise Kelly for supporting increased border resources.

But Kelly has weaknesses of his own as a vice-presidential pick. Biden and Harris have underperformed in Arizona vs. Trump so far, casting doubt on whether Kelly could deliver a swing state. He also only has four years of political experience, none of it executive, and some union leaders have raised concerns that he is not sufficiently pro-labor.

So far, Kelly has been less present on the campaign trail and cable TV than some of the other vice-presidential contenders, including Gov. Josh Shapiro of Pennsylvania, who co-headlined a Harris event Monday in Ambler, Pa.

Since his name was floated, packs of reporters have trailed Kelly around the Capitol and given him the opportunity to audition for the traditional veep attack dog role — even as he declines to say whether he wants the job. Instead, Kelly has shifted the focus to Trump’s own running mate, Sen. JD Vance (R-Ohio), whom he called “backward” for his opposition to supporting Ukraine in its fight against Russia.

“I think it’s one of the most important issues that we’re going to face here in a generation and what happens in Ukraine is going to affect us in our national security,” Kelly saidlast week.

Kelly is slated to appear on MSNBC’s “Morning Joe” on Wednesday morning ahead of Vance’s swing through Arizona, which will take him to Glendale and later the border.

Kelly and Harris have spoken since Biden announced he was stepping aside, according to a person with knowledge of the phone call who spoke on the condition of anonymity because they were not authorized to discuss it publicly. Harris is slated to pick her running mate before Aug. 7, when the Democratic Party will nominate its presidential ticket using a virtual process.

“Vice President Harris has directed her team to begin the process of vetting potential running mates,” a Harris campaign spokesperson said. “That process has begun in earnest and we do not expect to have additional updates until the vice president announces who will be serving as her running mate.”

Kelly’s wife, former congresswoman Gabrielle Giffords (D-Ariz.), appeared in Grand Rapids, Mich., for Harris on Monday. After suffering a near-fatal gunshot wound in 2011, Giffords became a leading activist against gun violence, with Kelly co-founding a national gun-control group that carries his wife’s name.

While not known as a particularly energizing public speaker, Kelly has been in high demand as a surrogate for other Democrats this campaign cycle, including Harris before she was elevated to the top of the ticket. Several of his Senate colleagues are eager to be seen with him.

“She is, without a doubt, one of the best, most bipartisan senators in the United States Senate,” Kelly said while appearing at a rally for Sen. Jacky Rosen (D-Nev.) in June.

He’s a prolific fundraiser and is battle-tested after winning two fiercely contested Senate races by margins of roughly two to five percentage points, first during a special election in 2020 and then in 2022, when he secured a full six-year term.Republicans threw a mountain of opposition research at him both times, trying to cast suspicion on his transformation from Giffords’s husband to a wealthy politician with business ties to China. Kelly won both times and had — until recently — been enjoying the freedom of not running for reelection, he has told colleagues.

In the Senate, the Arizona Democrathas focused on national security issues; he serves on both the Intelligence and the Armed Services committees. He was also a part of the negotiations that resulted in the bipartisan Chips and Science Act, which boosted manufacturing of semiconductors in the United States to counter China’s influence.

Kelly also bluntly clashed with Sen. Tommy Tuberville (Ala.) when the Republican held up military promotions over objections to allowing service members to be reimbursed for travel related to abortions, saying it didn’t make “sense” that Tuberville, who is not a veteran, insisted he was the most “military” person on the Armed Services Committee.

But it is Kelly’s style on immigration that some Democrats think would make him a unique asset to the presidential ticket.

Much like Arizona’s other senator, Kyrsten Sinema, an independent, Kelly long has distanced himself from Democrats’ handling of border security. During his 2020 campaign, Kelly frequently told supporters that more border security and resources were necessary to aid towns dealing with a flood of asylum seekers. He noted that the debate over undocumented immigration and resources in Arizona had been going on for decades — through Democratic and Republican administrations alike.

“When Democrats are wrong, like on the border, I call them out on it, because I’m always going to stick up for Arizona,” Kelly said during a 2022 campaign debate.

Despite speaking differently than many Democrats on the border, Kelly’s legislative record is slim on the issue given Washington has been virtually deadlocked on it. Sinema, a former immigration lawyer and veteran dealmaker, has often taken the lead on Washington’s border efforts, including the failed bipartisan compromise to crack down on border crossings. Kelly supported the deal but was not deeply involved in crafting it.

Additionally, Kelly co-sponsored legislation aimed at helping border officials combat fentanyl trafficking that became law earlier this year, and helped pass a measure with Sinema aimed at boosting agents who patrol tribal lands for drug smugglers.

Kelly slammed his Republican colleaguesfor voting against the compromise after Trump opposed it, calling it a “shameful” day for the Senate. “This isn’t just a political talking point for me or my state. It’s a reality we live every single day,” he said.

It’s that sort of blunt assessment of Washington’s attitude toward the issue that could carry broad appeal in both Rust Belt and Sun Belt states, said Adam Kinsey, an Arizona Democratic consultant.

“He understands how [the border] operates, how it works, he understands the challenges, the pressure points based on who he represents,” Kinsey said. “It’s more real, more visceral for him than it is for a senator who isn’t on the border.”

Republicans are skeptical Kelly can boost Harris on the border. “Regardless of how much they try to say she was not the czar, she was clearly leading point on this issue,” said Sen. Mike Rounds (R-S.D.). “I don’t think on that particular policy issue that she can get away from it.”

Back home in Arizona, Kelly has built a powerful political brand in the swing state, where he performed well with Latino voters in his 2022 race, a key constituency for Harris as she plots a path to victory in 2024. Sixty-eight percent of Latinos backed his candidacy, according to an analysis by the progressive-voter-file company Catalist. That was higher than the 63 percent of Arizona Latino voters who backed Biden-Harris in 2020.

“In 2022, he beat every statewide [candidate] on the ticket by several points,” said Catalist adviser Haris Aqeel,referring to his performance with all voters.

Kelly, who lives about an hour north of the border in Tucson, has cultivated a reputation for accessibility on border issues, and is well known among border officials. He often flies himself around border communities in the vast desert state to both get in pilot hours and visit his constituents and elected officials.He stresses the need for more resources and better enforcement while also speaking about the need for comprehensive immigration reform to provide a path to citizenship for some undocumented immigrants.

“He’s well educated, well versed on the topic,” said Yavapai County Sheriff David Rhodes,a Republican. “He’s been a good conduit of information and he is approachable — he does get back to us.”

Rhodes contrasted that with Harris.

“We haven’t heard a single word from Vice President Harris,” Rhodes said, referencing the diplomatic role Biden gave her to address the root causes of migration from Central America. “I don’t know exactly what she was told to do, but we have not seen or heard from her on the border in Arizona.”

Tyler Pager contributed to this report.