By any measure, Democrats are on the defensive in the next fight for Senate control. A three-seat Democratic midterm gain, which would give the party a majority, looks virtually impossible given the seats up this cycle.

A net change of eight seats would be large by historical standards but not unprecedented. Swings of at least eight Senate seats have occurred in four of the last 17 midterm elections — 1958, 1986, 1994 and 2014 — and in six of the last 34 elections (going back to 1950).

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The problem for Republicans is that these big Senate swings have always happened against the sitting president’s party. The sole exception, since the direct election of senators, occurred in 1934, when President Franklin Roosevelt’s party gained 10 Senate seats. Two years earlier, when Roosevelt won a landslide presidential victory, his party gained a dozen Senate seats.

The sitting president’s party has gained Senate seats in only four of the past 17 midterms, and each time the gain has been minuscule — one seat in 1970, 1982 and 2002, and two seats in 1962.

History, then, is not on the GOP’s side.

But since 2016 was something of a “black swan” election and Donald J. Trump remains a wild card, it’s probably premature to dismiss the possibility that 2018 could produce another unusual outcome.

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The only Republican Senate seat at risk as the cycle begins is in Nevada. GOP freshman Dean Heller was elected in 2012 when he squeezed by Democrat Shelley Berkley in a photo finish, 46 percent to 45 percent. This year, Democrat Hillary Clinton carried Nevada narrowly in the presidential race, so you can bet Democrats will go after Heller with everything they have.

A number of Republican senators could find themselves facing primary challenges, including Jeff Flake of Arizona and Ted Cruz of Texas. Utah’s Orrin Hatch may retire. But at this very early point, there is no reason to believe that any GOP-held seat other than Heller’s will be at risk, even if there are Republican retirements or messy primaries.

On the other hand, five Democratic senators in the class represent states normally classified as anywhere from leaning Republican to strongly Republican: Indiana, Missouri, Montana, North Dakota and West Virginia. Two other states are pure swing states: Florida and Ohio. And three states often lean Democratic but were carried by Trump last month: Pennsylvania, Michigan and Wisconsin. (Both Pennsylvania and Wisconsin currently have one Republican senator.)

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Joe Donnelly (D-Ind.) was a surprising winner in 2012 after popular veteran incumbent Richard Lugar (R) was upset in the GOP primary and the eventual Republican nominee, Richard Mourdock, stumbled through an underwhelming campaign.

Claire McCaskill (D-Mo.) looked headed for defeat four years ago until her GOP challenger, Todd Akin, started talking about rape in a way that offended many in the Show Me State. She won reelection easily against the inept Republican.

Jon Tester (D-Mont.) and Heidi Heitkamp (D-N.D.) proved to be strong campaigners who localized their races, while Joe Manchin (D-W.Va.) emphasized his conservative views on issues from guns to coal in a state that has become very Republican in federal elections.

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Debbie Stabenow (D-Mich.), who won by 21 points in 2012, seems like a difficult target, but Robert Casey Jr., who won by nine points, and Tammy Baldwin (D-Wis.), who beat a former GOP governor by only five points, appear to be softer incumbents.

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As always, the crucial question in any election is turnout — and the makeup of the electorate. That will be doubly so in 2018, since overall turnout drops during midterm elections. Midterm electorates usually look more Republican, with whites and older voters constituting a larger percentage of voters.

Since Trump won the electoral vote while losing the popular vote by more than two percentage points and 2.8 million votes, his victory was all about the combination of states he won. If he can energize his supporters in normally swing states and even Democratic-leaning states he carried this year, he could have a huge impact on the Senate midterm elections.

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Since the president-elect is still weeks from being sworn in, it’s impossible to know how he will be viewed in 2018. If he is widely seen as successful, he could turn out to be a considerable asset for Republican Senate hopefuls in states with competitive contests.

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If he is seen as a disappointment, or worse, voters will use the midterm elections to send a message of dissatisfaction to him and his party. In that case, Trump would be an albatross around the neck of Republican nominees in states that he was not expected to carry in 2016, including Pennsylvania, Michigan and Wisconsin, as well as in states where the Republican label is not a disadvantage.

Trump voters’ strong commitment to the president-elect suggests that they’ll stand by him over the next two years. He has shown an ability to blame others (the media, Democrats, etc.) when he gets into political trouble, and that could help him rally supporters in 2018, even if his job approval numbers are relatively weak and a majority of Americans are unhappy with the direction of the country.

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Election cycles invariably look very different when they begin and when they end. Retirements, primary fights and results, political events in the nation’s capital, and economic and foreign-policy developments all impact voters. We simply can’t know what voters will be thinking 23 months from now.

Nevertheless, the Senate already bears watching — not to see whether Democrats can retake the body in 2018, which seems unlikely now, but to see whether Republicans can strengthen their hold on the Senate and strengthen the hand of President Trump.