Everything you need to know about the 2018 midterm elections

In every midterm election since the Civil War, the president’s party has lost, on average, 32 seats in the House and two in the Senate.

In next year’s battles, Democrats need only 24 seats to flip the House and two to take the Senate.

“History says we’re going to lose the majority,” said Cory Bliss, the executive director of the Congressional Leadership Fund, a major Republican super PAC. “Our job is defy history.”

But the GOP majorities are defended not just by incumbency and super PACs, but by structural advantages in both chambers.

“We remain in prime position to defend our majorities in 2018,” said Republican National Chairman Ronna McDaniels, in a statement to NBC News.

In the Senate, the battleground offers far more liabilities than opportunities for Democrats because the 33 states in play next year are redder than average.

“We’re going to have a headwind, there’s no question about that,” said Rob Jesmer, the former executive director of the National Republican Senatorial Committee. “The question is, does the map bail us out?”

For instance, in 2012, House Democrats won about 1 million more votes nationwide than Republicans, but that wasn’t big enough to put them anywhere near retaking the chamber.

Experts disagree on exactly how big of a landslide Democrats need in the House — estimates range from as little as 53 percent to as much as 58 percent of the national vote — but they agree a narrow majority like 2012’s won’t cut it.

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