Sen. Mark Kelly, a potential vice presidential contender, said on Wednesday that he would vote in support of the PRO Act, a major labor reform measure.

The remarks came a day after ABC News reported concern among labor unions about the Arizona senator as a potential running mate for Vice President Kamala Harris due to his unwillingness to back the pro-labor legislation.

"Why would the Democrats even consider a senator for the vice presidency if he doesn't support the PRO Act," John Samuelsen, President of the Transport Workers Union, told ABC News.

On Wednesday, Kelly voiced full support for the PRO Act, saying he would vote for it if it came to the Senate floor.

"I would have voted for it on Day 1," Kelly said. "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 Huffington Post first reported Kelly's comments on the PRO Act. Kelly spokesperson Jacob Peters confirmed the comments to ABC News.

Kelly, who took office in 2020, has declined to sign onto the PRO Act throughout his tenure. The latest version of the bill, known as the Richard L. Trumka Protecting the Right to Organize Act, boasts the support of 48 of the 51 U.S. Senators who caucus with Democrats.

Kelly and Pennsylvania Gov. Josh Shapiro are reportedly the two leading candidates for the nod as vice president on a potential Harris-led ticket, a senior administration official told ABC News on Tuesday.

Prior to Kelly's remarks on Wednesday, a union president who backs Harris told ABC News that they opposed the nomination of Kelly due to his position on the PRO Act. The union president requested that ABC News not use their name due to the sensitivity of issues surrounding Harris's candidacy.

"The Democrats cannot have someone on the ticket who is identified as soft on labor," the person said. "This is a huge problem."

In a statement on Tuesday, Kelly's office said the Arizona senator has robustly backed labor.

"Senator Kelly is the son of two union police officers and has been a strong supporter of workers throughout his time in the Senate," Peters told ABC News.

Peters pointed ABC News to a statement Kelly made to the Huffington Post in 2021 in which he said he supported the "overall goals" of the legislation while acknowledging that he had "some concerns."

In 2022, Kelly's Senate campaign was endorsed by the Arizona AFL-CIO and the Association of Flight Attendants-CWA.

Larry Cohen, former president of the Communications Workers of America, previously said Kelly's position on the PRO Act raised concern but the primary consideration in assessing his selection as vice president should be whether he helps the Democratic ticket win in November.

"I would tend to go back to the issue of the swing states. Who can move the needle?" Cohen told ABC News, noting Arizona is a battleground state. "The number one goal is beating Trump."