It’s no surprise that campaign ads for Mike Braun, the Republican candidate for Senate in Indiana, feature President Trump. More striking: Democratic incumbent Joe Donnelly’s ads do, too.

The fight for the Senate is being fought on a deep red, pro-Trump battlefield that is dramatically different from the suburban landscape where the House majority will be won and lost.

Ten of the Senate Democrats up for re-election this year represent states that Mr. Trump won in 2016—five of them by 20 percentage points or more. By contrast, Hillary Clinton won 23 GOP-held districts considered competitive this year.

That is why Democrats’ chances for erasing Republicans’ narrow 51-49 Senate margin are longer than their prospects of winning the House majority, which would require them to flip 23 seats.

In recent weeks, the Senate map has shifted for both parties: Democrats are worrying more than expected about deep-blue New Jersey, where Sen. Bob Menendez—whose trial on federal corruption charges ended last fall in a hung jury—faces a well-funded GOP opponent, Bob Hugin.