By Robert Romano

Just as midterm elections tend to favor the opposition party in races in the House — 89.7 percent of the time the incumbent party loses seats, with losses averaging about 36 seats — so do presidential elections tend to favor the White House incumbent party.

In presidential election years dating back to 1900, the White House incumbent party picks up seats in the House 57 percent of the time, with gains averaging 18.7 seats.

If Republicans were to pick up seats and win the average number of seats gained in 2020 — it looks like they’ll have about 200 seats in the next Congress — they’d be well within striking distance. President Donald Trump and the GOP could take back the House. It’s not impossible.

It would be the first time since 1948 that the White House incumbent party held 1600 Pennsylvania Avenue and flipped control of the House. So, there’s that.

And 57 percent of the time is hardly a sure thing.

On the flip side of the equation, of the 13 times House seats were lost in a presidential cycle, 9 of those were years the White House incumbent party lost the election, in some cases catastrophically. In 1932, Republicans lost 101 seats in the House for example.

So, if the GOP loses House seats in 2020, there’s a 69 percent chance Trump would lose re-election.

Overall, in years the incumbent party lost the White House and seats in the House, losses in the House averaged 34.5 seats. Suffice to say, that would be a catastrophe for the GOP.

Interestingly, the White House incumbent party usually loses seats in the Senate during presidential election years, 57 percent of the time, with losses averaging 4.3 seats.

However, losing Senate seats is not as fatal and is far less predictive to presidential reelection hopes as losing House seats. In years that Senate seats were lost, the White House incumbent party went on to win the election 59 percent of the time.

The size of Senate losses appears to depend on whether the White House was lost or not.

In years Senate seats and the White House were lost, losses averaged 8 seats.

In years Senate seats were lost but not the White House, losses only averaged 1.7 seats.

Also of interest, in years Senate seats were gained by the White House incumbent party, 61.5 percent of the time that party kept the presidency.

Meaning, if Trump gets re-elected in 2020, the odds are that the GOP would keep the Senate — and could even possibly win back the House.

And given how well Republicans did in the Senate, picking up a net 2 seats, Trump could be very well positioned for 2020.

In midterm elections going back to 1906, White House incumbent parties have only picked up Senate seats 6 times: 1906, 1914, 1962, 1970, 2002 and 2018. After that, White House incumbent parties have proven to be unbeatable, sweeping to victories in 1908, 1916, 1964, 1972 and 2004. No president during that period has lost re-election after picking up seats in the midterms. You have to go back 1888, when incumbent President Grover Cleveland lost to Benjamin Harrison after picking up 2 seats in the Senate. Note: In 1906 and 1886, senators were still selected by state legislatures.

The lesson is that if you’re a Republican member of either the House or Senate, or rooting for them, you sure better hope Trump gets re-elected or at least keeps things close. Their fortunes more or less are joined at the hip.

Robert Romano is the Vice President of Public Policy at Americans for Limited Government.