Political strategists say the main risk to Mr. Bannon’s strategy is that Mr. Trump’s popularity with Republicans is singular and nontransferable. Those voters most loyal to Mr. Trump may not show up to vote for Republicans running for Congress, they say, because they have no interest in midterm elections.

In fact, data that Republican pollsters have analyzed over the last year shows that Republicans are already staying home.

But Mr. Nunberg said the call to arms for Republican voters would be a worthwhile message.

“Donald Trump is the threshold issue of this midterm whether the Republican consultant class likes it or not,” Mr. Nunberg said. “Unlike the Democrats in 2014, who mistakenly ran away from President Obama, we need to embrace President Trump and set the terms. Jobs, economy, national security and impeachment. 2018 is about saving the Trump presidency. That is a winning message.”