“Even if you don’t call it a litmus test, this is what defines us: a willingness and a courage to make bold changes — not to just follow when it’s all said and done,” she said.

The impact on the race has already been palpable. Candidates who might have dodged questions on hot-button policies in the past are now finding themselves discussing once-audacious ideas in public forums, sometimes awkwardly so if they are caught off guard.

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Lesser known candidates are also using the emerging issues as an opportunity to differentiate themselves, as Mr. Buttigieg did with his response on court packing.

“My purpose is to get people thinking about what an ambitious agenda actually looks like,’’ Mr. Buttigieg said in a telephone interview this week, “and to remind everybody that structural reform is an option — whether it’s this, or the Electoral College, or any of the other stuff.’’

The former housing secretary Julián Castro called for a federal study of reparations for black Americans, even as several top-tier candidates waffled on their support, and during an interview on CNN over the weekend Mr. Castro called out Mr. Sanders by name for declining to support reparation payments. Gov. Jay Inslee of Washington has made the elimination of the Senate filibuster a major theme of his candidacy.

“While the world burns, the Senate will sit in total inaction because of an antiquated vestige of a bygone era,” Mr. Inslee said in an interview, referring to the filibuster. “We’re in an internet age and this is something from the steam engine.”