A source who has seen recent polling, conducted by the Republican National Committee, told me the data show that a majority of Trump voters don't believe the mountain of evidence that Democrats will win back the House in November.

By the numbers: 57% of strong Trump supporters believe it's unlikely Democrats win the House, according to the source, who wasn't authorized to share findings from the RNC poll with the media. (The survey of 800 registered voters — 480 via landline calls and 320 via cellphone calls — was conducted from Aug. 29 to Sept. 2 and has a margin of error of 3.5%.)

By contrast, election forecaster Nate Silver's FiveThirtyEight website gives Democrats an 83.1% chance of winning control over the House.

Why this matters: A month ago, we reported in "Sneak Peek" that Republican strategists were detecting something interesting — and from their POV, concerning — in focus groups of Trump voters. These voters — who have been listening to the president predicting a "red wave" in November — didn't believe polls showing Democrats would win the House.

This disbelief freaked out Republican strategists who want their voters to be panicked enough to vote in November.

But, but, but: Several Republican officials who have reviewed the latest polling tell me they see an opportunity amidst the gloomy data. They think they can energize seniors, suburban women and Republicans less likely to vote by attacking the high costs and potential implications of Democrats' "Medicare for All" single payer health care plans.

For these Republican constituencies, an anti-Medicare for All message "is the best performing message of persuading and motivating these groups in this November’s election," said a source familiar with the president's political thinking.

"If you're giving something to everyone, that means less for seniors," the White House's political director, Bill Stepien, told me.

Trump has been briefed on this new polling and has been hammering these messages in recent rallies.

Democrats plan to fight back against this messaging. Democratic strategist Jesse Ferguson told me: "For years, Republicans have been trying to make Medicare wither on the vine, so voters aren't going to believe they're now trying to save it. Even if Lex Luther put on a cape, people wouldn't start believing he's Superman."