For some Democratic primary contenders, the demands may be different, reflecting the political realities of trying to establish a foothold in what is currently an 18-person field. In addition to Ms. Warren, former Mayor Julián Castro of San Antonio told CNN on Friday that initiating impeachment proceedings would be “perfectly reasonable.”
The willingness of both contenders to support impeachment hearings amounts to a refusal to engage in the political gymnastics of their rivals — but also highlights the urgency they feel about their own campaigns.
Mr. Castro has struggled to gain ground and is still working to attract enough individual donors to qualify for the first Democratic debate in June. Ms. Warren has remained in the middle of the pack despite entering the race before any other major candidate and unveiling an array of ambitious policy proposals.
Ms. Warren has also sought to distinguish herself by refusing, at least in the primary, to appeal to major donors. This has increased pressure on her to raise money online from small-dollar givers.
And in the hours after she announced her support for impeachment, she made the most of it, appearing on Rachel Maddow’s MSNBC show, the preferred programming of Mueller-minded Democrats, and posting a clip of it on her social media accounts.
By midday Saturday, Ms. Warren had promoted her new stance on Instagram three times in less than 24 hours, adding a video clip from an appearance in Keene, N.H., earlier that day. “There are some things that are bigger than politics,” she told the crowd there, to cheers. “This is one of them.”
But elsewhere on the trail, the Mueller report was little discussed. Campaigning in South Carolina after the document’s release, Senator Bernie Sanders held forth largely on affordable housing and the consequences of gentrification.
At last, on Saturday morning, Mr. Sanders posted a video targeting Mr. Trump on Twitter — for breaking promises to manufacturing workers in the industrial Midwest.