ANALYSIS — Political observers all know the party holding the White House tends to fare poorly during the midterm elections. That is why the combination of redistricting and the midterm dynamic clearly benefits House Republicans next year.

But reapportionment and redistricting don’t affect the Senate, and more importantly, a recently developing electoral dynamic — the surge in straight-ticket voting — might improve Democrats’ chances of holding the Senate in 2022.

The midterm dynamic

While the sitting president is not on the midterm ballot, voters often see these elections as opportunities to send a message about his first two years.

Those who are angry at the incumbent president and unhappy with his performance tend to turn out in big numbers to express their frustration and disapproval. Often, some who voted for him but are disappointed that he didn’t deliver on his promises stay home during the midterms, creating an electorate that is less favorable than the one that elected him just two years earlier.

Unlike the more visible, better-funded races for the Senate or governor, House members traditionally have found it more difficult to carve out their own identities and run independently of the president.

That is why it is so important to House members of the president’s party that he have strong approval ratings going into midterm elections. The president’s party has gained House seats in midterms only three times in the last 100 years: 1934, 1998 and 2002. All three qualify as “special circumstances” — during the Great Depression, during the Clinton impeachment controversy, and shortly after the 9/11 attacks.

There have been years when House losses by the president’s party were in the low or middle single digits — in 1962 and 1986, for example — but midterm elections have often produced dramatic losses for the president’s party, including in 1974, 1994, 2006, 2010 and 2018.

Large Senate swings have also occurred during midterms, including in 2014, when the GOP gained nine seats; in 2010 and 2006, when the party in the White House lost a half-dozen seats; and in 1986, when Republicans lost eight Senate seats during Ronald Reagan’s second midterm — a bigger defeat than the party suffered in the House that year.

But unlike the House, where every seat is up every two years, only one-third of the Senate is up every two years. That makes the states/seats that are up in a given cycle crucial in creating vulnerabilities for the two parties.

The end of ticket-splitting

While partisanship has run deep over many periods of American history, ticket-splitting has been widespread in American elections since the end of World War II.

More than 1 in 5 voters voted for the president of one party and a House member of a different party in every presidential election from 1956 to 1996, according to data compiled in “Vital Statistics on Congress.” Often, the percentage of voters splitting their tickets exceeded 30 percent.

That percentage started to slip in 2000, but it took a dramatic step down in 2012 (and stayed there in 2016 and 2020), when fewer than 1 in 10 voters split their tickets. In fact, only 16 House districts — a mere 4 percent — backed a Republican and a Democrat in their votes for president and the House last year, according to FiveThirtyEight elections analyst Geoffrey Skelley.

It wasn’t only House races that saw a sharp drop in ticket-splitting. Shortly before the 2020 elections, the Pew Research Center asked registered voters whether they would be splitting their tickets for president and the Senate and found that only 4 percent said they planned to do so.

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2022 factors

Next year’s election is a midterm, so all the data about presidential-year ticket-splitting doesn’t automatically apply. But the increasing partisanship and straight-ticket voting could be just as relevant in 2022 as it was in 2020.

Of the eight most likely competitive Senate seats this cycle, each party currently holds four.

The four Democratic seats are in states carried by Joe Biden last fall — Arizona, Georgia, New Hampshire and Nevada. Two of the GOP-held seats — Florida and North Carolina — are in states that Donald Trump carried, while two others — Pennsylvania and Wisconsin — are in states that went for Biden.

Given these numbers, straight-ticket voting should benefit the Democrats, while the midterm dynamic would benefit the Republicans.

But, of course, things are not that simple. States differ in their competitiveness, so there is a big difference between Biden carrying Wisconsin by less than a percentage point and New York by more than 23 points.

Biden carried Arizona, Georgia and Wisconsin by just a hair, Pennsylvania by just over a single point, Nevada by a couple of points, and New Hampshire by 7 points. On the other hand, Trump carried North Carolina by just over a single point and Florida by more than 3 points.

So while Biden carried six of the eight states with the most competitive Senate contests next year, his margins were generally small. That makes it difficult to argue that Democrats would benefit dramatically from straight-ticket voting in those states. It could help them, or it could hurt them, depending on what the national and state political landscapes look like as November 2022 approaches.

If, for example, progressives are so disgruntled with Biden’s first two years in office that they stay home, that would create a midterm electorate that could produce large House and Senate Democratic losses.

The nominees matter too, of course, which is why both parties are waiting to see whether New Hampshire Republican Gov. Chris Sununu will challenge Democratic Sen. Maggie Hassan next year. (North Carolina and Pennsylvania will be open seats, which adds another small twist.)

So there are many factors at play in the midterms, not merely a single dynamic. If Biden maintains a strong job approval rating and Democrats can make the 2022 elections “about” Trump and GOP extremism, Democrats certainly have a real shot to hang on to their Senate majority. But they have little room for error.