It all begins at 6pm EST, and the very first results will likely provide a clue how the rest of the night will play out.
The main event in the first states to close will be the Indiana Senate race, where Democratic incumbent Senator Joe Donnelly holds a slim lead in polling over Republican Mike Braun, but has only averaged about 44% in absolute support. A Republican Senate win in Indiana would make it very difficult for Democrats to take the Senate majority. The Indiana race could also provide a hint regarding the outcome of the Democratic-held Senate seat in Missouri, where the polling is even closer.
In the House, the main race to watch is KY-06, a Republican-held seat which most analysts rate a toss-up. A Democratic win here would signal Democrats are on track to win the majority.
Florida and Virginia are likely to be most in focus. An upset win by Republican Governor Rick Scott over Democratic Incumbent Senator Bill Nelson would all but ensure a Republican Senate majority.
In the House, if Democrats cannot win the Republican-held seat in FL-27, they might not be on track for a majority after all. On the other hand, if Democrats win FL-26, they could be on track for a slim majority, and if they win FL-15, they could be on track for a large majority. In Virginia, there are three bellwether races: If Republicans hold onto VA-10, they might have a very good night. If they hold onto VA-07, they have a shot at maintaining the majority. By contrast, a Democratic win in VA-02 would signal a more substantial Democratic majority is likely.
Incumbent Democratic Senators Brown (OH) and Manchin (WV) appear very likely to win reelection but an upset would all but guarantee a Republican Senate majority. In the House, if Democratic challenger Dan McCready can wrest NC-09 from Republican control, Democrats will likely be on track for a majority. Democratic wins in NC-02 or NC-13 would send just as strong a signal. Democratic wins in OH-01 or OH-12 would suggest a Democratic majority is likely as well.
The outcome of the Senate majority should start becoming clear by 8pm. If Democratic incumbent Senator Claire McCaskill wins another term, it would be difficult for Republicans to gain net seats and could, depending on how other races go, keep open the path to a narrow majority. That path would also likely run through Tennessee, where former Governor Phil Bredesen is challenging Rep. Marsha Blackburn for the open Republican-held seat. Short of winning in Tennessee, Democrats would need to win the Senate seat in Texas held by incumbent Senator Ted Cruz, which looks even more challenging. If Democrats cannot win Missouri and Tennessee (or Texas) they are unlikely to win the Senate majority.
In the House, if a "blue wave" has formed it should be apparent in the results from states with polls closing at 8:00pm. Four Republican-held seats are expected to flip to Democrats as a result of redistricting in Pennsylvania (PA-05, PA-06, PA-07, PA-17), with another expected to flip to Republicans (PA-14). PA-01 could be a bellwether; if Democrats cannot win in this Clinton-won district they might have trouble winning the majority. In New Jersey, Democrats look likely to pink up at least 3 or 4 seats (NJ-02, NJ-03, NJ- 07, NJ-11); if they cannot manage to win more than two, Republicans might hold onto the majority. In Texas, the outcome in TX-32 should be watched; that district narrowly supported Sec. Clinton in 2016 at the same time it voted for Republican Rep. Pete Sessions by more than 50%. A Republican loss here would signal a high probability of a Democratic majority.
At this point the general direction of the House elections should have become clear, but the Senate might still be in play. The main focus will be the Republican-held seat in Arizona, which Is the Democrats' best pick-up opportunity in the Senate this year.
Control of the Senate should become clear with the races that close at 10pm ET. Democratic Senator Heidi Heitkamp (N.D.) trails in most polling, but Democrats have a shot of offsetting a potential loss there with a pick-up in Nevada, where the race is close to tied. Incumbent Democratic Senator Jon Tester leads polling in his reelection effort, but with a fairly narrow margin.
If the House majority is shaping up to be a close call, the results in California and Washington state could be decisive. Democrats probably need to flip at least 3 or 4 of the Republican held seats in California and Washington to win the majority. This might not be too much of a challenge, given how many of these Republican-held seats voted for Hillary Clinton in 2016.
Source: Goldman Sachs