If you plan to watch election results roll in on Tuesday, here’s our first piece of advice: get comfortable.

Democrats need to pick up a net gain of 23 Republican House seats to win back the majority, which would be a critical balance-of-power shift halfway through President Trump’s first term.

A Democratic takeover is considered likely, but a bunch of the most hotly contested seats are in west coast time zones that guarantee results will not be in until long past midnight in the east. In addition, some races may be too close to call, and voting procedures in states on both coasts may put off a final tally for several days.

Republicans are much less likely to lose control of the Senate, and they may broaden their majority in that chamber. If Democrats have any chance to take back the Senate, they have to win two east coast races that are more likely to be settled before Wednesday dawns.

Here’s our guide of how to watch on election night, and key races to keep your eyes on. Sorry to say, there are no Massachusetts races in this list, because none of the major races in the state are looking particularly competitive.

Early Warning Signs (8 p.m.- 10 p.m.)

WGBH Radio kicks off our live election coverage at 8, so find us on the air!

Northeastern states will provide a first glimpse of whether Democrats really have a chance to win the House.
One of the true “toss-up” races is Maine’s huge 2nd Congressional District — covering basically all of the state above Portland and Augusta. Republican Rep. Bruce Poliquin has held this seat for four years, but he is in a neck-and-neck race with Democratic challenger Jared Golden. There are also two third-party candidates who are unlikely to win, but who could affect the outcome of the vote.

Maine is also using a new “ranked voting” scheme that means if one candidate does not win over 50 percent of the vote, a complex game of Election Survivor begins, with the lower performing candidates being eliminated and their votes redistributed to the remaining candidates. The Portland Press Herald has a nice explainer of this — and why it may be a while before we know the winner.

One of the most entertaining congressional seats in New England is New Hampshire’s 1st District, where Democrat Carol Shea-Porter and Republican Frank Gunita have swapped the office several times since 2007. Neither one of them is running this year, but it still is one of the nation’s swingiest swing districts, with Democrat Chris Pappas running against Republican Eddie Edwards (NOT the former Louisiana governor of the same name). Libertarian Dan Belforti is also on the ballot but barely registers in local polling. This is currently a Democratic held seat.

There are several other Republican seats in jeopardy in the Northeast, including a handful in New Jersey. GOP Reps. Frank LoBiondo (NJ-2) and Rodney Frelinghuysen (NJ-11) are retiring, and their districts are considered prime pickup targets for Democrats. Incumbents Tom McArthur (NJ-3) and Leonard Lance (NJ-7) may also be in trouble, according to the non-partisan election handicapper Nathan Gonzales of Inside Elections.

Those races could give an early indication of whether a Democratic wave is building.

However, longtime New Jersey political reporter Herb Jackson points out that the Garden State has changed its voting law to rely more heavily on mail-in ballots, and those ballots are accepted up to 48 hours after the polls close if they were postmarked by Election Day. That means in a close race, it could be days before a result is final, and litigation may ensue. A county election clerk explained this process in a letter to the Asbury Park Press in October.

Pennsylvania could also be a big turning point for Democrats on Election Day, but it is wildly confusing in part because the state Supreme Court redrew the boundaries of the state’s congressional districts this summer, leaving everybody running in new turf.

Gonzales said he is looking at Pennsylvania's 1st District race as a bellwether for Democrats: First-term Republican Rep. Brian Fitzpatrick is running for reelection in a suburban Philadelphia district that has been re-drawn to include more Democratic voters. A recent New York Times poll shows Democratic challenger Scott Wallace with an edge, but the Times admits it was a small poll and not very definitive.

Also in the east: Keep your eyes on GOP Rep. Dave Brat of Virginia. Brat is a Tea Party hero who came out of nowhere in 2014 to upset then-Majority Leader Eric Cantor in a Republican primary in the Richmond-area 7th District. He is now in a dead heat with Democrat Abigail Spanberger in his bid for re-election, according to a recent Washington Post poll.

The Senate seats to watch in the east are Democratic incumbents Bob Menendez of New Jersey (tainted by a corruption scandal) and Bill Nelson of Florida, who are both in tossup races for re-election. If either of them lose, it would be almost impossible for Democrats to win back Senate control, even if West Virginia Democratic Sen. Joe Manchin survives.

Second Wave To Watch (10 p.m.- midnight)

The big focus in the middle of the country will be Senate races in Indiana (Joe Donnelly), Missouri (Claire McCaskill) and North Dakota (Heidi Heitkamp), where three endangered Democratic incumbents are in races that are too close to call. Democrats need to win them all to win back the Senate, and they still need to knock off a Republican somewhere. Maybe Ted Cruz in Texas? There has not been a poll yet in that race that shows Democrat Beto O’Rourke beating Cruz. Perhaps Tennessee is a better pickup opportunity, where polls indicate former Gov. Phil Bredesen and GOP Rep. Marsha Blackburn are in a dead heat for the Senate seat being vacated by moderate Republican Sen. Bob Corker.

There are also some key House races to watch mid-country. Republicans have two pick-up opportunities in Minnesota, as Democrats Rick Nolan (retiring) and Tim Walz (running for governor) are giving up their seats. If you hear Jim Hagedorn declared the winner, that’s a Republican pickup in Walz’s seat; in Nolan’s 8th District seat, Pete Stauber is the GOP contender.

In Illinois, watch the 6th District race of GOP Rep. Peter Roskam. He has been in Congress since 2007, but his district voted heavily for Hillary Clinton in 2016. He is being challenged by Democrat Sean Casten.

And then keep your eyes on Iowa. Republican Reps. Rod Blum and David Young are both in tight re-election races, and ultra-conservative Rep. Steve King suddenly looks vulnerable after 15 years in Congress because of recent incidents linking him to white nationalist groups.

Look West Late (wee hours Wednesday until ?)

Most likely, the west coast will be the deciding factor in House control, and those folks don’t even finish voting until 11 p.m. our time.

In California alone there are nine Republican House seats that are considered tossups or very competitive; obviously a close race takes longer to declare a winner and there are always possibilities for recounts or challenges that could drag the process out.

The big name to watch in California is Rep. Dana Rohrabacher — the idiosyncratic pro-Trump and pro-Russia conservative who has represented his district in Congress since 1989. He’s in the toughest race he’s ever faced against Democratic businessman Harley Rouda.

In Washington, all ballots are by mail, and ballots are valid if they are postmarked by Election Day, which means votes can still trickle in for several days. One of the most competitive races in the country is the 8th District, outside of Seattle, where seven-term moderate Republican Dave Reichert is retiring. Another small-sample New York Times poll found no advantage for either Democrat Kim Schrier or Republican Dino Rossi, so the counting may run late.

Also in the Evergreen State, Rep. Kathy McMorris Rodgers, the highest ranking Republican woman in the House, is facing a stiff challenge in the seat once held by former Democratic House Speaker Tom Foley. McMorris Rodgers is still favored in the heavily Republican district, which voted for President Trump by a large margin, but this is the kind of seat that could turn if there is a true "blue wave."

And if Democrats have managed to hold on to their endangered Senate seats in the eastern states, control of the Senate will hang on Senate races in Nevada and Arizona. In Arizona, Republican Jeff Flake is retiring and Republican Rep. Martha McSally and Democratic Rep. Kyrsten Sinema are running to replace him. In Nevada, GOP Sen. Dean Heller is running for re-election against Democratic Rep. Jacky Rosen. Both races are considered toss-ups.