A ripple, a wave or a tsunami? Here’s where to watch to know on election night.

I ask all my sources the same question — and it comes from the movie “Back to the Future Part II.”

In the picture, the hapless series villain, Biff Tannen, is visited by his future self, carrying a sports almanac that holds the result of games yet to be played. Altering history and ushering in a crime-ridden dystopia, Tannen goes on to build a fortune as the most successful sports gambler ever.

What I ask my sources to do is to imagine they’re confronted by an older man or woman bearing a peculiar resemblance to themselves. This strangely familiar person offers them a copy of a future edition of the “Almanac of American Politics,” but only under two conditions: They can only look at House districts — and they can only look for five seconds.

So they have to think fast: Of the 435 congressional districts, they have to pick one — the House seat that will say everything they need to know. Given the GOP is heavily favored to win the majority, the key is to pick the true bellwethers — not necessarily for which party will control the chamber, but for whether Democrats or Republicans will have a good election night, compared to expectations.

Here are five races that frequently come up as the contests the pros will be watching. Two are open seats, the purest tests of the political environment. The other three involve endangered incumbents: a Trump-district Democrat, a Biden-district Republican, and a two-term member who has been forced, by redistricting, to seek reelection in a seat where most of her constituents don’t live.

And all five are currently rated as “Toss Ups” in POLITICO’s Election Forecast. Here are the races to watch on Tuesday night — and beyond — to get the best sense of what kind of election we’re having.

Illinois’ 17th District

Eric Sorensen (D) vs. Esther Joy King (R)
Cook Political Report Partisan Voter Index: D+2
Estimated 2020 presidential result: Biden +7.8

The topline: An open seat in blue territory in northwest Illinois.

State legislative Democrats sought to shore up retiring Rep. Cheri Bustos’ seat in redistricting, transforming it from one then-President Donald Trump won by 2 points to one President Joe Biden would’ve carried by 8 points.

But that still might not be enough for Democrats to hold it. The race between former TV meteorologist Eric Sorensen and 2020 GOP candidate Esther Joy King is tight, and both parties are fully engaged in the ad battle to win it. The district takes in swaths of rural Illinois, in addition to the Quad Cities on the east banks of the Mississippi River — places where Republicans have been making gains recently.

County to watch: Rock Island. In 2020, Bustos beat King 57 percent to 43 percent in Rock Island, which includes Moline. That was down from Bustos’ 65 percent there in 2018 — and Sorensen can’t afford much more slippage than that.

Nebraska’s 2nd District

Rep. DON BACON (R) vs. Tony Vargas (D)
Cook Political Report Partisan Voter Index: Even
Estimated 2020 presidential result: Biden +6.4

The topline: What matters more — incumbency, or our increasingly nationalized politics?

GOP Rep. Don Bacon has survived the past few elections even as his Omaha-based district has drifted left. Though Trump won the seat in 2016, Biden comfortably captured its electoral vote two years ago.

For his part, Bacon has struggled to navigate Trump’s role as party leader. He voted against both impeachments, but he also crossed the aisle to back a commission to investigate the Jan. 6, 2021 attack on the Capitol. He also supported the 2021 infrastructure law — prompting Trump to later ask his followers, “Anyone want to run for Congress against Don Bacon in Nebraska?”

But Trump didn’t find any takers. Bacon won the GOP primary without breaking a sweat, though he now faces his toughest test in years. The Democratic nominee, state Sen. Tony Vargas has outspent the incumbent and is running on cutting middle-class taxes.

Bacon’s campaign as mostly been positive, though Republicans have labeled Vargas a liberal and sought to tie him to House Speaker Nancy Pelosi.

County to watch: Douglas. Most of the district’s voters reside in Omaha-centered Douglas County, where Bacon nearly ran even with his 2020 Democratic opponent, Kara Eastman, despite the fact Trump lost the county by more than 11 points.

Oregon’s 6th District

Andrea Salinas (D) vs. Mike Erickson (R)
Cook Political Report Partisan Voter Index: D+4
Estimated 2020 presidential result: Biden +13.7

The topline: The most Democratic-leaning district on this list, the race for Oregon’s new House seat is emblematic of a lot of the party’s blue-state woes.

Even though Biden would’ve carried the seat by 14 points, Democrat Andrea Salinas is locked in a tight race with Republican Mike Erickson.

The closer-than-expected toss-up race for Oregon governor — a seat Republicans haven’t won in 40 years — has contributed to making the House contest close. The GOP’s focus on crime appears to be paying dividends, especially in the Pacific Northwest, according to strategists in both parties.

If Tuesday is a good night for Republicans, they’ll pick off districts like this one and two others in Oregon. There’s also a similar district in California’s Central Valley, and two on Western Long Island.

What they all have in common: They’re open seats in blue states. Both parties agree that such districts are ripe for Republican takeovers this year.

County to watch: Yamhill. Yamhill is a Republican county, but Trump’s margin there over Biden in 2020 (4 points) showed some signs of slippage from 2016, when he beat Hillary Clinton by 8 points.

Pennsylvania’s 8th District

Rep. MATT CARTWRIGHT (D) vs. Jim Bognet (R)
Cook Political Report Partisan Voter Index: R+4
Estimated 2020 presidential result: Trump +2.9

The topline: Yes, 2022 is shaping up like a rough year for Democrats — but that doesn’t mean they’re going to lose all of their incumbents in seats Trump carried two years ago.

Democratic Rep. Matt Cartwright has been a survivor in Northeast Pennsylvania, winning by 4 points in 2020, even as Trump carried the seat by a similar margin.

He’s facing a rematch against Republican Jim Bognet this year — and while the new district lines got slightly more favorable for Cartwright, public and private polls still show a tight race.

In this polarized era, ticket-splitting voters are increasingly rare. And a strong Republican year could wipe out Trump-district Democrats like Cartwright, Rep. Jared Golden in Maine (Trump +6.3), and Tom O’Halleran in Arizona (Trump +8.4).

County to watch: Luzerne. In 2020, Cartwright edged out Bognet in Luzerne County, home to Scranton, 51 percent to 49 percent — even as Trump won Luzerne by a whopping 14 points. Flipping some of those Trump-Cartwright voters is an imperative for Bognet this year.

Virginia’s 7th District

Rep. ABIGAIL SPANBERGER (D) vs. Yesli Vega (R)
Cook Political Report Partisan Voter Index: D+1
Estimated 2020 presidential result: Biden +6.3

The topline: The seeds of Republicans’ midterm optimism were planted one year ago in Virginia.

That’s when the GOP’s Glenn Youngkin won the governorship, breaking eight years of Democratic rule and flipping a state that had voted for Biden by 10 points just the previous November.

Now, Youngkin is barnstorming the commonwealth — and outside of it, too, as he seeks a national profile — to build upon the momentum in congressional elections, focused in large part on ousting Democratic Rep. Abigail Spanberger.

Republicans nominated Prince William County Supervisor Yesli Vega. On paper, the district is a good bellwether. It’s partly suburban, stretching south from outer-ring suburbs and exurbs of Washington, D.C., into more rural areas.

Though Spanberger is an incumbent who’s raised a lot of money, she’s also running in a redrawn district that’s quite different from her current one, which is based farther south in the Richmond suburbs. More than three-quarters of the new district’s voters aren’t currently represented by Spanberger, though her TV ads have been running in the Washington media market for nearly five years now.

Biden carried the district by 6 points in 2020 under the new lines, which makes it friendlier territory than Spanberger’s old district (Biden +1.3).

And Spanberger’s been a strong candidate in the past: She was the only Democratic candidate in a race that POLITICO’s Election Forecast rated as a “Toss Up” in 2020 to win. Republicans swept all the other “Toss Up” seats as they outperformed expectations, especially further down the ballot.

One other bonus for election watchers: Unlike some other states, Virginia is expected to complete nearly all of its vote count on Tuesday night. That means even in relatively tight races, we should know who won before the night is done.

County to watch: Stafford. About 1-in-5 district voters live in exurban Stafford County, south of Washington. It’s new to Spanberger — and it’s generally competitive territory. Youngkin carried it by 11 points over Democrat Terry McAuliffe in 2021, but Biden narrowly beat Trump there in 2020, 51 percent to 47 percent.