Arizona Senator Mark Kelly soared quickly to the top of Kamala Harris’s list of potential running mates due to his record as a former astronaut and two-time winner in a key swing state.

As he rose, he drew flak from the same union bosses who helped the Democrats pick their nominee. They said his failure to co-sponsor the Protecting the Right to Organize (PRO) Act was a deal breaker.

“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.

The PRO Act is an umbrella bill that hands unions everything they’ve demanded to violate the rights of workers and employers who don’t want to be unionized. It would prohibit employers from holding anti-union meetings at work, ban state right-to-work laws, bar employers from permanently replacing strikers, and make California’s infamous anti-gig-worker AB5 national.

Faced with union pressure, Kelly buckled. The left-wing Huffington Post reports: “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.”

All of this is a reminder that when a Democrat runs on a presidential ticket, they must completely toe the party line. In 2000, the Harvard Crimson reported that when Al Gore picked Connecticut Senator Joe Lieberman as his running mate “all of Lieberman’s centrist leanings (had to be) ditched for the position of number two yes-man.”