Senate Republicans Are Newly Hopeful About the Midterms. For Good Reason.

A major question looming over the 2018 Senate contest is whether so-called wave election years — in which one party makes significant gains in both chambers of Congress, as happened in 1994 and 2006 — can still exist as the country grows more polarized and politics more shaped by hardening party preferences. With ticket-splitting fading, especially in federal races, voters are increasingly turning to lawmakers who reflect the presidential leanings of their state.

That could spell trouble for Democrats representing largely conservative electorates and states where surveys show that, unlike in much of the country, the president is viewed more favorably than unfavorably.

“In the middle of the country people are by and large center-right, and they see the national Democratic brand as really far left, which is a ball and chain those senators have to carry around,” Senator John Thune, Republican of South Dakota, said.

But Democrats argue that the well-cultivated reputations and financial advantages of party incumbents like Senators Heidi Heitkamp of North Dakota, Jon Tester of Montana and Joe Manchin III of West Virginia matter as much as the red-leaning nature of their states.

And they say that what passes for good news on the right — simply being competitive in states the G.O.P. otherwise dominates — underscores the Republicans’ weakness in a year when the map is so favorable.

“We’re feeling very good about our chances,” said Senator Chuck Schumer of New York, the minority leader, adding, “At a minimum, there’s a 50-50 chance we’re going to take back the Senate.”

There are only nine Republican seats in play, but Democrats believe they have the chance to win in three: Arizona, Nevada and Tennessee.