That’s because, if history is any guide, the congressional GOP should expect to lose control of the House and maybe even the Senate in two years, unless it boosts the former and future president’s popularity and the economy they will jointly oversee.

The reason is the age-old American political phenomenon known as “surge and decline.”

First comes the surge.

In presidential election years in the United States, the White House-winning party benefits in Congress from the extra turnout for the victorious candidate at the top of the ticket, a phenomenon known as “coattails.” The term originated about 400 years ago, referring to the rear flap of a man’s coat. Then, in an 1848 speech, a relatively unknown Whig Illinois congressman named Abraham Lincoln countered Democratic critics accusing Whigs of hiding behind the “military coattail” of their White House nominee, Gen. Zachary Taylor, charging that Democrats themselves had spent two decades hiding behind former President Andrew Jackson’s coattails.

A century later, a landmark book on predicting elections documented an average 26-30 House seat “coattail effect,” that is, the impact of winning presidential candidates sweeping in same-party members of Congress with them.

Around that time, the term began a meteoric rise in English language books, though it has since fallen off in usage, perhaps in part because critics of the theory began to suggest the effect was not as strong as originally thought. Regardless, the winning presidential candidate’s party has won 13 House seats and two Senate seats on average since 1932.

Then comes the decline.

Since at least the development of the two-party system in 1860, in midterm election years, the lack of the winning presidential candidate at the top of the ticket nearly always results in congressional-level losses for the party in the White House. That correction generally results in House and, to a lesser extent, Senate losses proportional to the size of the prior election cycle’s presidential victory. Specifically, for every percentage point of the margin by which a president wins in the prior election, his party will lose about three seats during the midterm elections.

At last count, Trump appears to have won about 49.9% to Harris’ 48.3%, a 1.6 percentage point margin. On average, then, we’d expect the House GOP to lose about five seats in 2026. That would cost it congressional control because, in the 119^th Congress, House Republicans will hold a slim 220-215 majority. The incoming Senate Republican majority, which will be 53-47 in the next Congress, also could be vulnerable in 2026, based on the same dynamic.

Where might Hill Republicans get help to avoid this type of scenario?

Historically, a handful of factors seem statistically to increase or decrease a presidential party’s midterm losses. Those factors include presidential popularity and, to a lesser extent, economic growth. Specifically, for every percentage point more favorable the president is in the midterm Gallup poll, one can expect the president’s party to save about one seat in the House, on average. Similarly, each one-percent improvement in economic conditions in the year before the midterm elections saves the president’s party about two seats in the House, though there’s a question of whether that dynamic holds true in the Senate.

As a result, if history is any guide, congressional Republicans’ best hope to retain control of their respective chambers would be to make Trump as successful, and especially well-liked, as possible during the next election season. Secondarily, the House GOP, in particular, can improve its chances by seeing to it that the U.S. economy is thriving to the maximum extent possible before that election season arrives — in this case, 2025.

Granted, statistically speaking, a third factor may affect those losses: the degree to which individual members of Congress position themselves strategically relative to the president. If a president is unpopular, some members of his party might choose to distance themselves from him. That said, it only works on the margins, as Democrats running under retiring President Joe Biden’s yoke can testify — and if Trump is popular and the economy is roaring, GOP members won’t have that problem. So, on average, congressional Republicans can either make Trump popular and economically successful — or, if the past is prologue, lose power.

The bottom line: Midterm math may mandate that congressional Republicans help Trump succeed or fail themselves.

Christopher C. Hull, Ph.D. is president of Issue Management Inc., a public affairs firm that does grassroots and advocacy work. He was previously chief of staff to a member of the House of Representatives.