Sen. Mark Kelly (Ariz.), a contender to become Vice President Kamala Harris’ running mate on the Democratic presidential ticket, told HuffPost on Wednesday that he would vote in favor of the pro-union legislation known as the Protecting the Right to Organize Act.

Kelly’s lack of full commitment to the PRO Act, as it’s known, has stood as a possible barrier to him becoming Democrats’ vice-presidential nominee, since labor unions are a crucial part of the Democratic coalition. Harris has become the presumptive nominee since President Joe Biden withdrew from the race on Sunday.

“Why would the Democrats even consider a senator for the vice presidency if the senator doesn’t support the PRO Act?” John Samuelsen, president of the Transport Workers Union, told ABC News.

But Kelly on Wednesday made clear in an interview with HuffPost that he would support the bill if it came to the Senate floor, affirming enthusiastic support for labor unions. He is not listed as a co-sponsor on the Senate’s most recent version.

“Unions loom large in our life, and I’m supportive of the PRO Act,” Kelly said, recounting how when his mother, a police officer, was injured, her union helped her recover.

“I would have voted for it on Day 1,” he added of the bill. “I would vote for it today. I am, like a lot of legislation, working to make it better. But if it came to the floor today or any day going back to the day I was sworn in, I would vote for it.”

The PRO Act amounts to a sweeping overhaul of labor law that Democrats believe would make it easier for workers to form unions and bargain with their employers.

Among other measures, the legislation would create stiff financial penalties for illegal union-busting; ban anti-union “captive audience” meetings at work; make it easier for newly unionized workers to secure their first contracts; nullify state right-to-work laws; and bar employers from permanently replacing strikers.

But the bill has not drawn unanimous support within the Democratic caucus.

Kelly previously expressed concerns to HuffPost in 2021 regarding a provision that would extend collective-bargaining rights to “independent contractors” — a measure many business groups strongly oppose.

At the time, Kelly said he would “like to see some changes” to the bill, saying only that he was open to passing certain parts of the PRO Act through a process known as reconciliation.

“Depending on how it’s done, I’m not necessarily opposed to that,” he said then.

Kelly is among several Democrats being mentioned as Harris’ potential running mate. The former fighter pilot and astronaut has a strong resume and could help Democrats win Arizona, a key swing state. He’s been a reliable Democratic vote in the Senate, and his vocal embrace of the PRO Act now, amid speculation about higher office, could be a sign of his interest in the job.

The senator declined to answer multiple questions earlier this week about whether he’s been asked by Harris’ campaign to submit documents vetting his potential candidacy, a prerequisite for any candidate.

“Kamala Harris is going to be the nominee, she’s the right person for this job, and I’m gonna work really hard to make sure that we don’t see Donald Trump back in the White House,” is all Kelly would tell HuffPost on Tuesday.

Other Senate Democrats heaped praise on Kelly as a vice presidential contender on Wednesday, highlighting his NASA background.

“Obviously, I know Mark Kelly, I think he’d be phenomenal,” Sen. Cory Booker (D-N.J.) said. “Not because he’s a centrist but because he’s an extraordinary leader that is literally out of this world.”