Despite Trump's low ratings, midterm election map still favors Republicans — for now

Key Points
  • History shows that a president's party often loses ground in Congress during midterm elections.
  • Results are even worse when the president's Gallup approval rating is below 50 percent before Election Day. Trump's currently stands at 37 percent.
  • The current electoral map, however, bucks historical norms, showing trends that stand in the way of Democrats taking control of Congress.
Lawmakers walk out of the US Capitol in Washington, DC.
Nicholas Kamm | AFP | Getty Images

The caution in every investment prospectus applies to politics, too: "Past performance is no guarantee of future results."

Congressional Republicans desperately hope not.

Fifteen months before the 2018 midterm elections, key indicators from past elections look bleak. That heaps additional pressure on Republican lawmakers as they struggle to work effectively with an erratic, embattled new president of their own party.

Start with modern electoral history. In the 18 midterm elections since World War II, the president's party has lost an average of 25 House seats and 4 Senate seats.

Then narrow that group to the midterm elections in which the president's party controls both houses of Congress, and therefore can't easily deflect responsibility for problems onto the opposition. In those nine mid-term elections, the president's party has lost an average of 33 House seats and 4.7 Senate seats.

In an overlapping group of nine mid-term contests, the president's approval rating in the Gallup Poll was below 50 percent immediately before Election Day. In those elections, his party did even worse, losing an average of 36 House seats and 5.8 Senate seats.

Wisconsin: Trump's approval rating slips in state that flipped red in election
Wisconsin: Trump's approval rating slips in state that flipped red in election

Seven modern midterm contests have combined both factors: unified control of both ends of Pennsylvania Avenue and presidential approval below 50 percent. And they paint an even more ominous picture for today's Republican majority in Congress.

In those seven midterms, the president's party lost an average of 41 House seats and 6.4 Senate seats. In four cases, control of the House flipped; in three of them, control of the Senate flipped, too.

Next November, a gain of 30 seats would give Democrats control of the House. A gain of three seats would give them control of the Senate.

President Donald Trump's Gallup approval rating now stands at 37 percent.

Using traditional prediction formulas, notes Congressional elections scholar Gary Jacobson, "no way the Republicans hold onto the House." Even if Democrats didn't win the Senate, that would roadblock Trump's legislative agenda.

But the changing political landscape has diminished the value of some old guideposts. That provides Republicans some comfort 200 days into the Trump presidency.

First, partisan voting patterns have grown more consistent as Democrats and Republicans have grown increasingly polarized in recent decades. That diminishes ability of Democrats to attract dissident Republican voters.

Second, the combination of political gerrymandering with the residential concentration of Democrats in large metropolitan areas gives Republicans the ability to win a share of House seats far outpacing their share of the overall population. So the current Democratic lead in national "generic ballot" polls – a robust 9 percentage points in the current realclearpolitics.com average – exaggerates the potential for Democratic gains.

Third, today's Democratic Party has grown increasingly reliant on younger voters. They traditionally turn out for midterm elections at lower rates than their elders.

In the Senate, moreover, Republicans hold a huge advantage benefit in the profile of seats up for election next November. Of 34 senators on the ballot, only nine are Republicans; of the seven considered most vulnerable, only two are Republicans.

As a result, says fivethirtyeight.com analyst David Wasserman, "the Congressional map has a record-setting bias against Democrats."

Congressional Republicans need all the help they can get. Not only is Trump historically unpopular for a president in his first year, but the all-Republican government has failed to deliver on any of its major promises to voters.

The seven-year GOP crusade repeal and replace Obamacare collapsed before the president and Congress left town for summer vacation. When they return in September, party leaders face the urgent task of convincing balky lawmakers to protect America's credit-worthiness by raising the federal debt ceiling.

The White House and Congressional Republicans vow to deliver by year-end on their pledge to cut rates and overhaul the nation's tax system. If they fail, they may not have the chance to try again soon.

WATCH: Majority want Congress to move on from health care

Majority of Americans want Congress to move on from health-care reform: Reuters/Ipsos poll
Majority of Americans want Congress to move on from health-care reform: Reuters/Ipsos poll