Those five were the senators who spoke at the 2020 audition last month, and they have engaged in a monthslong policy shadow dance, each signing onto others’ proposals while zipping to the front of the line with fresh, liberal ideas of their own.

Ms. Gillibrand has signed onto legislation to decriminalize marijuana (a Booker bill), agreed on the need to cut interest rates for indebted student-loan borrowers (currently Warren legislation, though Ms. Gillibrand had pushed a similar measure before), and pushed a bill to strengthen unions (along with Mr. Sanders). She was the first to endorse aloud a jobs guarantee, while Mr. Booker has introduced a pilot program and Mr. Sanders is promising more sweeping legislation to come. On postal banking, Mr. Sanders and Ms. Warren endorsed the idea first; Ms. Gillibrand was the first to put it into legislation.

When Ms. Gillibrand forswore corporate PAC money in February, she was following Mr. Sanders and Ms. Warren; Mr. Booker joined them within hours. (Ms. Harris, at a town hall, initially waffled on doing so. “It depends,” she said. But she soon reconsidered and joined the rest of the bloc by late April.)

For Ms. Gillibrand, it represented a stark reversal for a politician who raised nearly $5 million from business PACs earlier in her career, and was confronted in 2013 by John Oliver on “The Daily Show” about her substantial contributions from the likes of Goldman Sachs and JP Morgan Chase & Company.

“What is required of you, for that money?” Mr. Oliver asked. “Because it makes me uncomfortable.”

Today, Ms. Gillibrand, who has long called for publicly financed campaigns, says that her decade-plus in Washington has taught her that “every ill in Congress, no matter what it is, it will stem from the fact that money corrupts politicians and politics.”