The record of midterm elections shows the power of the president's standing clearly and consistently. In each of the last three such contests, more than 80 percent of those approving of the president's performance back his party's House candidates; more than 80 percent of those disapproving of the president voted against his party.

In several surveys in recent days, the proportion of Americans who approve of Trump's job performance has fallen back below 40 percent. Voters intensely hostile to the president far outnumber intense supporters. And without Trump on the ballot, a dangerous number of those intense supporters may not even show up to vote.

This week's Quinnipiac University poll showed that just 38 percent approve of Trump's performance while 54 percent disapprove. If those numbers hold over the next eight weeks, Republicans struggling to hold their House and Senate majorities will be fighting steeply uphill.

The disparity in enthusiasm between the two sides makes it even steeper. Recent special elections have shown Democrats more motivated than Republicans to vote.

A CNN poll this week measured an identical imbalance in intensity: 27 percent of voters strongly approve of Trump, while 48 percent strongly disapprove. Trump himself, addressing supporters in the White House recently, worried aloud about what that means.

"There's a real question as to whether people are going to vote if I'm not on the ballot," the president told a gathering of conservative Christians.

Republicans have always known they'd face a challenging political climate in November. Any president's party almost invariably loses House seats.

Their hope had been that a robust economy would keep their House losses below the 23 seats Democrats need to seize control. Though Democrats need to gain only two seats to take over the Senate, several Democratic incumbents must defend their jobs in strongly pro-Trump states such as North Dakota and West Virginia.

Earlier this year, Republicans felt growing encouragement. The strong economy helped lift Trump's approval ratings into the low- to mid-40s and narrowed the Democratic advantage in the national surveys gauging voter preferences for control of the House.

But now Trump's unpopularity provides Democrats a path to victory for both chambers. It is overpowering satisfaction with the economy and widening the Democrats' "generic ballot" edge.

"The situation looks more worrisome for Republicans," says GOP pollster Kristen Soltis Anderson. One particular vulnerability is among the young voters whose attitudes are Anderson's specialty; in the Quinnipiac survey, voters 18-34 disapprove of Trump by a 2-to-1 margin.

The Cook Political Report now lists 66 Republican-held House seats as in serious danger of flipping. Polling averages on realclearpolitics.com show Democratic Senate candidates currently leading for three Republican-held seats — in Arizona, Nevada and Tennessee.

History paints an especially ominous picture. Since World War II, presidents with job approval ratings below 50 percent in the Gallup Poll have lost an average of 36 House seats. In each of the last three midterms — 2006, 2010 and 2014 — the president's party has lost six Senate seats.

In the last half-century, only President George W. Bush in 2006 has suffered pre-midterm approval ratings as low as Trump's recent sub-40 percent levels. That year, Democrats ousted Republicans from control of both the House and Senate.

CORRECTION: This article has been updated to show that this week's Quinnipiac University poll showed that 54 percent disapprove of Trump's performance.