This year, Republicans generally succeeded in recruiting high-performing candidates to Senate contests in Florida, North Dakota and Arizona, even in a national political environment that sent House Republicans for the doors.

Democratic House candidates were helped by the declining value of incumbency, which made it harder for Republicans to outrun disapproval of the president.

The same forces, however, made it harder for Democratic senators to run as far ahead of the national party as they had in the past, and often their states had shifted far to the right since their last election.

In general, the split decision between the House and the Senate can be attributed mainly to the combination of the growing relationship between presidential vote and congressional vote and the declining value of incumbency. The apparent loss of Senator Bill Nelson in Florida, in particular, is not consistent with a typical wave election result. But this is the main driver of the difference between the results in the two chambers.

Democrats benefited from a huge number of Republican retirements, and they have flipped eight of those seats so far. Many retirements were inevitable, but the number — the highest since 1992, a redistricting year — was not. Democrats also benefited from a string of court decisions that eroded or outright eliminated Republican gerrymanders in Florida, North Carolina, Virginia and, most recently, Pennsylvania.

It is hard to measure the accumulated effect of these decisions. But it could have easily represented the Democratic margin of victory in Virginia’s Seventh District and in Pennsylvania’s Fifth, Sixth, Seventh and 17th. That’s atop Democratic gains already realized in 2016 in Florida and Virginia.

The Democratic disadvantage in the Senate isn’t going anywhere. State lines aren’t about to be redrawn, after all, and Hillary Clinton won just 19 states in 2016 while winning the national popular vote.