The campaigning for Tuesday’s midterm elections has been going on since spring. If you tuned it out months ago, here’s a quick summary of what’s at stake:

1. What’s a midterm election?

A national election to select members of Congress, held around the halfway point of a president’s term. This year’s midterms will be held Tuesday, Nov. 4. Voters will choose all 435 members of the House (plus the District’s one non-voting delegate), and 33 members of the Senate.

Also on Tuesday, elections for governor will be held in 36 states, including close contests in Maryland and Florida, and a mayoral election in the District of Columbia.

2. Why aren’t all 100 senators running for re-election this year?

Because of the Constitution. Article I, Section 3 specifies that there should be three “classes” of senators, serving staggered six year-terms, so that only one-third of the Senate faces re-election at a time. The reason, according to the Senate’s historians, was to provide stability. Even when huge “wave” elections swept away incumbents, some of the old Senate and its ideas would remain. “As the federal government’s only continuing body, the Senate could provide leadership after major elections and during other periods of national uncertainty,” the Senate historical office says.

3. How many House and Senate races are competitive this year?

Surprisingly few. In the House, expert handicappers say that about 85 percent of the seats are in no danger of flipping from one party to the other. Of the rest, only about 25 – or about 5 percent – are considered true toss-ups.

The lack of competitive races is partly the result of rampant “gerrymandering,” a practice in which state legislatures draw up congressional districts to pack them with voters from one party or the other. (The Founding Fathers had wanted this chamber to swing with public opinion. But then again, it was one of the Founding Fathers – Elbridge Gerry of Massachusetts – who invented the gerrymander.)

In the Senate, there is more drama than there is in the House. Of the 33 Senate seats up for grabs, 20 are considered safe. Among the remaining 13, three seem certain to change from one party to the other: West Virginia, Montana and South Dakota, where Democratic senators are retiring and Republicans are expected to take their place. That leaves 10 races where both parties have at least a small chance of winning the seat: Alaska, Arkansas, Colorado, Georgia, Iowa, Kansas, Kentucky, Louisiana, New Hampshire and North Carolina.

4. Which parties control the House and the Senate right now?

Republicans control the House, where they have held a majority of seats since 2011. Republicans occupy 233 House seats, which is 15 more than the 218 they need to hold the majority.

Democrats have controlled the Senate since 2007. Democrats hold 53 Senate seats, and two independent senators – Bernard Sanders (Vermont) and Angus King (Maine) – caucus with Democrats.

5. Who will control the House and the Senate when the election is over?

In the House, Republicans will still be in charge and probably will gain at least a handful of seats.

In the Senate, the most likely outcome seems to be that Republicans will win a majority and take control in January, but that outcome is still in doubt.

6. If Republicans win the Senate, what will happen then?

Not a lot, in the grand scheme of things. Republicans will take over the chamber’s committee chairmanships, and Sen. Mitch McConnell, R-Ky., will become majority leader (assuming he wins his re-election fight). But many bills require 60 votes, and President Barack Obama has the power to veto legislation.

There also are signs that the Senate Republican mix of conservatives, moderates and tea partyers may have difficulty getting along.

7. On election night, when will we know who won?

Probably not until very late, or maybe not at all.

Here’s the best-case scenario: By 9 p.m. Eastern time Tuesday, polls will have closed in eight of the 10 states with crucial Senate races. If either side wins enough seats decisively in those states, the night’s drama will be over early.

But there are several reasons to think that probably won’t happen. For one thing, the rules in Georgia and Louisiana require that a runoff election be held weeks later if no candidate wins a majority. And in Kansas, independent Senate candidate Greg Orman hasn’t said whether he would caucus with Republicans or Democrats if he defeats Sen. Pat Roberts (R).

So it’s possible that the battle for the Senate might still be unresolved at 1 a.m. Eastern time, when polls close in the last key state, Alaska, which has a long history of slow election tallies. In 2008, the result of the Senate race there wasn’t clear until two weeks after Election Day.