Forget the House. It’s the battle for the Senate that could provide the most drama on election night.

If control of the Senate comes down to a race-by-race contest, Republicans have a slight upper hand. To gain the majority, Democrats need to protect the five incumbents in the reddest states and win two of four competitive races where Republicans hold the seat. Each defeated incumbent makes ultimate victory significantly more difficult.

Right now, several Democratic incumbents are in jeopardy, starting with Sen. Heidi Heitkamp of North Dakota, who is being challenged by Republican Rep. Kevin Cramer. Heitkamp won her last race with just 50.5 percent of the vote. Republicans express more confidence about this contest than almost any other. The Cook Political Report lists it as a toss-up.

Not far behind in degree of difficulty for the Democrats are the races in Missouri and Indiana, where Democratic Sen. Joe Donnelly faces Republican businessman Mike Braun. Republicans like to recall they were given little chance initially of defeating former senator Evan Bayh in his 2016 comeback attempt, and they like their chances in this race. But some Republicans note that Donnelly is running a smart campaign.