WASHINGTON — Oddly, it has been business as usual on the campaign trail this week. Senator Elizabeth Warren explained her policy plans to voters in New Hampshire on Wednesday, Senator Bernie Sanders walked a picket line with striking auto workers in Detroit and former Vice President Joseph R. Biden Jr. courted donors in Bel Air, Calif., and went on Jimmy Kimmel’s late-night TV show.

But behind the scenes, many of the 2020 Democratic candidates have been grappling with a new reality: For the first time, a presidential primary will unfold with a sitting president seeking re-election while also facing an impeachment inquiry.

There is no political playbook for this. No pollster or strategist to speak from experience. What is clear, from interviews with advisers and allies with the various campaigns, is that the top candidates like Mr. Biden and Ms. Warren believe they are sufficiently established in the race that they can continue to command attention for their candidacies and messages. Second-tier and third-tier candidates, however, are at much greater risk of falling off the radar screen for an electorate that may soon be at a standstill, distracted by an impeachment circus in Washington.

“This is a nuclear bomb dropped in the middle of the race and it’s going to change the dynamics for everyone,” said Jared Leopold, an aide to Gov. Jay Inslee of Washington, who dropped out of the race several weeks ago. “You can’t go on pretending it was like it was a week ago.”