Two House Democrats facing challenging re-election races in rural districts said on Tuesday that President Biden would lose in November to former President Donald J. Trump, adding to widespread pessimism within the Democratic Party about its presidential nominee.

The two Democrats, Representatives Jared Golden of Maine and Marie Gluesenkamp Perez of Washington, did not call on Mr. Biden to step aside, nor did they indicate that any other Democrat stood a better chance of defeating Mr. Trump in the fall.

But Mr. Golden and Ms. Gluesenkamp Perez, who are some of the most vulnerable incumbents in Congress this election cycle, essentially delivered a warning that they were preparing for Mr. Biden to be a critical liability at the top of the ticket. A poor performance by Mr. Biden in the presidential election could doom their own chances for re-election.

In an opinion essay in The Bangor Daily News, Mr. Golden, who represents a district that Mr. Trump won in 2020, said that he had assumed Mr. Trump would win for months now and that he had made his peace with that outcome.

“Lots of Democrats are panicking about whether President Joe Biden should step down as the party’s nominee,” he wrote. “Biden’s poor performance in the debate was not a surprise.”

“It also didn’t rattle me as it has others,” Mr. Golden added, “because the outcome of this election has been clear to me for months: While I don’t plan to vote for him, Donald Trump is going to win. And I’m OK with that.”

In an interview with a local television station, Ms. Gluesenkamp Perez, a first-term Democrat from a rural red district, also predicted that Mr. Biden would lose, blaming his dismal debate performance.

“About 50 million Americans tuned in and watched that debate,” she told KATU News, appearing crestfallen throughout the interview. “I was one of them for five very painful minutes.”

After carefully considering her response to a question about whether the president should step aside, Ms. Gluesenkamp Perez said: “The truth, I think, is that Biden will lose to Trump. I know it’s difficult, but I think the damage has been done.”

She added that Democratic primary voters had already chosen Mr. Biden and that “a core tenet of democracy is that you accept the results of an election.”

“Biden is the nominee,” she said.