“Republicans have a very nasty habit of snatching defeat from the jaws of victory,” Mr. Ryun added. “But sitting here a year out, I’m not sure you could ask for better dynamics.”

Republicans pointed to several moments during the two debates when they said Democrats had undercut themselves in surprising ways, particularly on issues that will be important to voters in competitive Midwestern states like Michigan, where the debates were held.

Mr. Biden, for instance, declared that his energy plan would end fossil fuel use — a position that seems highly risky not only in Michigan, the cradle of the American automobile industry, but in coal-producing states like Pennsylvania and Ohio that Mr. Trump won in 2016.

Republicans said that other leading Democratic candidates like Ms. Warren, whom Trump campaign officials now view as the strongest debater in the field, missed opportunities to speak broadly to the concerns of middle-class voters and instead focused on issues with narrow appeal to progressives in the Democratic base.

Asked on Tuesday night whether her health care plan would be paid for by raising taxes on the middle class, Ms. Warren did not answer, saying only that “middle-class families are going to pay less out of pocket for their health care.”

“No one talked about the middle class or cutting taxes for the middle class,” said Alex Conant, a Republican consultant who advised Senator Marco Rubio of Florida on his 2016 presidential campaign. “If anything, they talked about how much to raise taxes on the middle class. It’s bizarre how little they talked about issues that matter to most people.”