Next Tuesday, Democrats and Republicans will compete for the 42nd time in a nonpresidential-year contest — a rivalry that goes back to 1854. That’s the oldest such partisan competition in the world.

And despite the complaints of today’s Democrats that the Constitution is biased against them, Democrats have won Senate majorities 16 times versus 10 times for Republicans since senators started being elected by popular vote in 1914.

Similarly, in the 164 years of midterm Republican-Democratic House contests, Democrats have won control of the House 24 times to 17 for Republicans. In other words, over the long run of history, both parties have adapted to changes in opinion and issue focus in multiple realignments.

But over that time, each has retained its basic character. The Republican Party has been centered around a core of people thought to be typical Americans, but who are never a majority: Northern Protestants in the 19th century, married white Christians today.

The Democratic party has been a coalition of disparate groups not thought to be typically American but who together have often formed an American majority: white Southerners and Irish immigrants in the 19th century, blacks, Hispanics, and gentry liberals today.

President Trump’s candidacy in 2016 jumbled the partisan lies a bit. His stands on immigration and trade differed from both parties’ nominees over the last 60 years. His brash demeanor and disdain of polite norms cost him the support of some upscale Republican voters and won over similar numbers of downscale Democrats. But the absolute number of switchers was not large by historical standards.

Opinion today seems to stand much where it did two years ago. Trump got 46 percent of the popular vote then; his job approval today is 44 percent. Good economic numbers have made little difference to an electorate with no memories of the Great Depression but with vivid, seething grievances stoked by culture war politics.

Republicans seem poised to gain seats in the Senate, and Democrats are likely to win enough House seats to gain a majority there. But that’s more a matter of the political lineup than of changed opinion.

Newton’s Third Law of Motion says that for each reaction there is an equal and opposite reaction. The enthusiasm of downscale voters that switched 100 mostly Midwestern electoral votes to Trump in 2016 has been mirrored by the enthusiasm which impelled dozens of Trump detractors to run for Congress, and thousands of contributors, big and small, to send them record amounts of money.

The result is that Democrats, after long decrying money in politics, have become the party of big money. (Note how they’d like to repeal the Trump tax bill’s limits on deductibility of state and local taxes, which benefits only those with incomes over $350,000.) They’re outspending Republicans in almost every seriously contested race. Shrewd spending (and record Republican retirements) has made them competitive in districts long safely Republican.

Today, the Cook Political Report lists 100 Republican-held House seats as competitive. That’s almost half the 241 seats Republicans won in 2016. With Republican enthusiasm increasing to Democratic levels since the Kavanaugh nomination, it seems both sides will have robust turnout. Democrats probably won’t match the usual wave-year mark of winning half the seats they’ve targeted. But they only need a net gain of 23 for a majority.

Still, prognosticators Charlie Cook, Nate Silver, and Larry Sabato and the betting markets believe there’s a nontrivial chance they won’t. That’s evidence of the continuing strength of the partisan lines that have mostly prevailed since 1994 and were altered only marginally in 2016.

The likely Republican Senate gains are more a function of which states’ senators are up for re-election than of any change in opinion. Democrats are defending 26 seats, including 10 in Trump states; Republicans are defending only 9, just one in a Hillary Clinton state.

Democrats are running two attractive candidates in Trump states — former Tennessee Gov. Phil Bredesen, a genuine moderate, and Texas Rep. Beto O’Rourke, the liberal skateboarder whose rapturous journalistic profiles somehow failed to mention that his father-in-law is a billionaire real estate developer. But both seem unable to overcome the partisan tide.

Republicans are in good position to pick up Senate seats in North Dakota, Missouri, and Indiana, and possibly also Montana and Florida. In their two vulnerable seats, they’re leading narrowly in Nevada polls and trail narrowly in Arizona.

There will be a tendency to project Tuesday’s results into the future. But they look more like an attempt, by both parties, to relitigate 2016.