Portside

It wasn’t necessarily the night of either party’s dreams. The Democrats are poised to gain around 35 seats after Tuesday’s elections. Republicans seem likely to gain a few seats in the Senate, and they triumphed in some high-profile governor’s races.

But Democrats faced formidable structural disadvantages, unlike any in recent memory. Take those into account, and 2018 looks like a wave election, like the ones that last flipped the House in 2010 and 2006.

In the House, where the Democrats had their strongest showing, it’s impressive they managed to fare as well as they did. In a sense, Republicans had been evacuated to high ground, away from the beach.

At the beginning of the cycle, only nine Republicans represented districts that tilted Democratic in the previous two presidential elections. Even in a wave election, these are usually the only incumbents who are standing on the beach with a greater than 50 percent chance to lose.There were 24 such Republicans in 2006, and 67 such Democrats in 2010.

Democrats had so few opportunities because of partisan gerrymandering and the tendency for the party to win by lopsided (and thus inefficient) margins in urban areas. It gave Republicans a chance to survive a hostile national political climate that would have doomed prior parties. By some estimates, Republicans could have survived while losing the popular vote by nearly a double-digit margin.

The Democratic geographic disadvantage was even more significant in the Senate this cycle. There, Democrats were defending 10 seats carried by the president, including five that he won by at least 18 points.

As a whole, the House Democratic candidates overcame all of these disadvantages. They are on track to win more seats than Democrats did in 2006, with far fewer opportunities. They even managed to win more seats in heavily Republican districts than the Republicans managed to win in heavily Democratic districts in 2010.

Democrats pulled it off with an exceptionally deep and well-funded class of recruits that let the party put a very long list of districts into play. In prior years, the party in power wouldn’t have even needed to vigorously contest many of these races.

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.