Vice President Kamala Harris has received a flurry of endorsements from many of the nation's largest labor unions since she announced her candidacy for president.

Concern has emerged within the labor movement, however, over the potential selection of Sen. Mark Kelly, D-Ariz., as a running mate because he has not signed onto a key piece of labor reform legislation.

Current and former union officials told ABC News that the possible selection of Kelly sounds alarm bells due to his unwillingness to back the PRO Act, legislation that would ease the path toward forming unions and winning labor contracts. Some officials outright oppose the pick, while others say the policy position should be part of a wider assessment of Kelly.

At least one labor leader who backs Harris said Kelly's position on the measure should not reflect on his support toward labor or deter his selection as vice president.

Kelly and Pennsylvania Gov. Josh Shapiro are the two leading candidates for the nod as vice president on a Harris-led ticket, a senior administration official told ABC News on Tuesday. Harris is the presumptive Democratic presidential nominee after receiving more than half of the party's delegates.

"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 and an ally of President Joe Biden, told ABC News. "It's the most important piece of national legislation workers have right now."

The Transport Workers Union is an affiliate of the AFL-CIO, a 12.5 million member union federation that endorsed Harris on Monday. Samuelsen, who said he did not attend the meeting at which the AFL-CIO endorsed Harris, will not decide on his union's endorsement of Harris until after she selects her vice presidential nominee.

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.

Richard Bensinger, former organizing director at the AFL-CIO, the nation's largest labor organization, said in a post on X that he opposes Kelly due to his position on the PRO Act.

"Only 3 Democrats refused to sign on to the Pro Act, one of whom was Mark Kelly," Bensinger said on Sunday, after Harris announced her candidacy. "The right to organize unions is the most important thing to labor so that's a hard no."

In a statement, 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," Kelly spokesperson Jacob Peters told ABC News.

Peters pointed ABC News to a statement Kelly made to the Huffington Post in 2021 in which he said he supports "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, among other unions.

At least one labor leader whose union endorsed Harris told ABC News that the position taken by Kelly on the PRO Act should not reflect negatively on his perceived support for labor or deter Harris from selecting him for vice president.

The legislation has stood well short of passage in both chambers during Kelly's tenure in office, the person said. In turn, the person added, Kelly has chosen to prioritize his standing among voters in the highly contested state that he represents. The labor leader requested that their name not be used due to the sensitivity surrounding the Harris campaign.

"I think that Kelly voted against the PRO Act when he felt his vote was not going to make a difference but might have been necessary in order to achieve labor's bigger objective, which was to control the U.S. Senate," the labor leader said.

"We don't question his support for working people," the labor leader added.

A union president who backs Harris, however, told ABC News they oppose the potential selection of Kelly on account of his position on the labor reform measure. The union president requested that their name not be used due to the sensitivity surrounding the Harris campaign.

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

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

"I wouldn't rule out Mark Kelly but he certainly wasn't quick to support what I would call moderate labor reform in a democracy near the bottom in terms of workers' rights," Cohen told ABC News.

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

The AFL-CIO did not immediately respond to a request for comment. Nor did the Service Employees International union, the nation's largest private sector union, which endorsed Harris.