Most wave elections, such as the Democrats’ victories in 2006 and Republican gains in 2010 are in non-presidential years. Exceptions were in 1964, when Lyndon Johnson trounced Barry Goldwater, and in 1980.

An encouraging note for Republicans: The only time in the modern era that an incumbent party ran successfully for a third presidential term, 1988, the makeup of Congress remained virtually the same.

But even a mini-wave could affect the Senate, where 24 of the 34 seats up for election are held by Republicans. The Democrats need a net gain of at least four seats to take control.

It might take a tsunami to capture 30 seats and win the House. All 435 seats may be in play, but with redistricting and population patterns, Democrats have to win the overall vote by seven percentage points to take it back, David Wasserman of the Cook Political Report estimates. The latest NBC/Wall Street Journal poll showed Democrats with a four-point advantage in the general matchup.