What exactly are midterm elections?

In the most literal sense, midterm elections are exactly what their name suggests: the elections that take place halfway through a president's four-year term. That covers a huge range of contests, from congressional seats all the way down to small-town mayoral races and county sheriffs.

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U.S. congressional elections, which take place every two years, are the most high-profile contests. In the House of Representatives, the lower chamber, all 435 seats are contested every time. In the Senate, the upper chamber, senators' six-year terms are staggered so that roughly one-third of them are up for election each time. This year, 35 of the 100 Senate seats are up for grabs. All of these term lengths and election dates are set by the U.S. Constitution.

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When people refer to midterm elections, those congressional votes are usually what they’re thinking of. They offer voters their biggest opportunity in between presidential elections to weigh in on a president’s performance and shift the balance of power if they’re unsatisfied.

There are also thousands of other races taking place across the country. A majority of U.S. states — 36 of 50 — will hold elections for governor this year, and the vast majority of state legislatures have elections as well. In many places, voters will also choose judges, sheriffs, mayors and any number of other local officials. All of these elections are governed by state and local laws, not the U.S. Constitution.

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State and local races often go relatively unnoticed, even within the United States itself. But governors and state lawmakers wield considerable power, as can big-city mayors. The laws they create for their residents can vary greatly from place to place and sometimes serve as test cases for various policies.

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Perhaps even more critically, most states put their legislatures in charge of redrawing the boundaries of the state’s congressional districts (more on that later). This year’s elections will determine which party controls many of those legislatures — and the governors who can approve or veto them — when the process next takes place, in 2020. At every level of American government, the effects of midterm elections can last for years.

Which elections are the most important to watch? We’ll start with the House of Representatives.

While there are certainly some compelling individual races to watch, the easiest way to keep track of things is probably by looking at the “generic ballot.” This is when pollsters simply ask voters whether they prefer Democrats or Republicans to win control of Congress.

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Unlike in many democracies, that number has no effect on any actual races in the United States. But it is an important indicator of which party is likely to come out on top: A party that leads the generic ballot by eight percentage points, as Democrats do at the moment, won’t get exactly 8 percent more seats, but it’s still probably going to win more than the other side overall.

If voters heavily favor Republicans, for example, many districts that normally have a small or moderate advantage for Democrats will suddenly be vulnerable or vice versa. If you hear people talk about a “wave election,” as they have this year, that’s what they’re talking about — the fact that many more elections are competitive than normal because of the political mood.

All of this means that Democrats are the large favorites to win back the House: The news website FiveThirtyEight estimates that they have a roughly 85 percent chance of having the majority of seats come November. But, as was the case in 2016, that still means Republicans have paths to victory as well.

What about the Senate?

Only a third of the Senate comes up for reelection at a time, which means major political swings that can cause big shifts in the House are rare in the upper chamber.

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Instead, control of the Senate is usually determined simply by which states happen to be electing senators that year. If elections are up in more reliably Democratic states, Democrats will probably have a good year and vice versa. This year, the map favors Republicans by a large margin: Democrats must defend 26 of the 35 seats up for election, many of which are in states that voted for Donald Trump in 2016. Republicans can focus on winning back seats from Democrats in favorable places rather than trying to shore up their own senators.

So while Democrats would have to make a net gain of just two seats to control the Senate, which is split 51 to 49 in the Republicans' favor, their path to do so is much harder. As Washington Post political reporter Amber Phillips wrote in September, “Democrats will have to have a near-perfect run” of the toughest races. Here are her three most critical ones to watch:

North Dakota: Sen. Heidi Heitkamp is the most likely Democrat to fall. In a heavily Republican state, she is extremely vulnerable to losing her seat — perhaps especially after she voted against the confirmation of controversial Supreme Court nominee Brett M. Kavanaugh. Recent polls have shown Heitkamp trailing significantly. If she loses, it will be a big blow to Democrats' chances of regaining the Senate.

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Nevada: This is the Democrats' best opportunity to take a seat from Republicans. Nevada has voted for Democrats in the past three presidential races, and a Democrat won the open Senate seat that was up for election in 2016. Nevada’s population is also more than a quarter Latino, and pro-Democratic labor unions play a major role in the state’s politics. Right now, the race is nearly an even toss-up, with Republican Sen. Dean Heller perhaps slightly favored.

Indiana: This is another place where Democrats need one of their senators to hang on in a Republican state. Sen. Joe Donnelly appears in better shape than his counterpart in North Dakota: Polls give him an advantage. If he loses, it could be a sign that Republicans are going to have a better night than forecasters anticipated.

How will the results of the midterms affect Trump?

When people ask this question, they’re probably really asking about one thing: impeachment. But while impeaching Trump is a rallying cry for his harshest critics — and, according to polling, is surprisingly popular in general — it’s unlikely to happen.

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Impeachment is actually a two-step process, laid out in the U.S. Constitution. First, the House of Representatives considers the accusations against the president; if it votes in favor of impeachment (it requires just a simple majority), it means that the president has been formally charged. To actually remove him from office, the Senate must then vote to convict him on those charges, which requires a two-thirds majority. Presidents can be impeached without actually being kicked out of the White House, as happened to Bill Clinton in 1998.

All of this would be a possibility only if Democrats controlled both chambers of Congress, which is unlikely. Even if Democrats win both, their leaders seem lukewarm about impeachment; Nancy Pelosi, the top Democrat in the House, has said that impeaching Trump is “not a priority.”

What Democrats could do is launch more serious investigations into Trump’s conduct regarding Russia, his business dealings or any other matter that they believe Republicans have downplayed. That could turn up damaging new evidence, or it could simply dominate the news as Trump gears up for his reelection campaign. No matter what, though, expect gridlock. If a Democratic House and Republican Senate are battling each other, it’s hard to see any major legislation being passed.

For many people overseas, the U.S. electoral system appears strange — and seemingly unfair in how it represents the will of voters. Can that system be changed?

The question of changing the system usually comes up when people talk about presidential elections. U.S. presidents are elected by an indirect system called the electoral college, not by a simple popular vote. The person who gets the most total votes can still lose the presidency, as Hillary Clinton did.

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The electoral college plays no role in any other elections, including the ones taking place Tuesday. But, like the rules that govern congressional elections, it’s part of the U.S. Constitution. Changing those rules requires amending the Constitution rather than just passing a new law, and the bar for amendments is high. They have to be approved by two-thirds majorities of both chambers of Congress, and then approved again by at least 38 states (there’s also another way, by calling a new constitutional convention, but it has never happened before).

With the country deeply divided, getting two-thirds of either house of Congress to agree on major changes would be virtually impossible. But it’s not just partisanship that makes changes unlikely.

The status quo gives smaller states a big advantage. Seats in the House of Representatives are determined by population — the more residents a state has, the larger its share of the 435 seats. But every state gets two Senate seats no matter what. Small states, regardless of partisanship, aren’t likely to back amendments that would reduce their power.

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There’s more room for experimentation at the state level. States can’t change how they’re represented in Congress, but they can change how their own residents vote. Instead of using the first-past-the-post system — i.e., whoever gets the most votes wins, even if the top candidate doesn’t reach a majority — some states require top-two runoffs so that winners get more than 50 percent of the vote, and Maine uses a different system called instant-runoff voting. In reality, though, most states have laws and constitutions that mimic the national government.

What about those strangely shaped districts? Why do they exist?

Again, the answer is (at least partly) in the U.S. Constitution. The Constitution requires that the country take a census every 10 years and that seats in the House of Representatives be divided among the states in proportion to their population. So, once a decade, some states will either gain or lose seats depending on population shifts. The changes are determined automatically according to a mathematical formula.

But the Constitution doesn’t say how states should draw up the boundaries of those seats; each one is free to use its own method. Many states let their legislatures do it, which is how you get the infamous practice of “gerrymandering” — creating districts that make little geographical sense to give one party an advantage. The party in power at the start of a decade will obviously try to give itself more seats, either by connecting its own voters to one another in odd ways or by jamming as many opposing voters into the smallest number of seats as possible.

Those maps may later be challenged in court for being grossly unfair and even redrawn by judges. Partisanship and chaos are common.

Several states have taken steps to change this. California and Arizona have independent commissions that draw up their maps, and lawmakers and public officials are barred from taking part. Three other states have advisory commissions, which draw up recommendations for their legislatures to pass. Four more have appointed committees, where both parties can appoint members to take part. All of these are attempts to make redistricting a little less partisan and a little more fair.

Why is so much money spent on elections?

Federal law sets limits on how much people can donate per year to candidates for federal office (Congress or the presidency), political parties or political action committees — groups that use the money to support candidates. Here’s a list of those limits.

So if you want to give directly to the candidate or party of your choice, you can’t give that much. The catch is that there are virtually no restrictions if you want to support a candidate indirectly. The law allows certain kinds of outside groups to accept unlimited amounts of money and use it to advocate for or against a candidate. The most famous kind of those groups are “super PACs,” which have become a way for wealthy donors from both sides to dump money into elections.

Technically, those independent groups aren’t allowed to coordinate with the campaigns or parties they support. But the definition of “coordination” is narrow, and super PACs are closely tied to candidates and parties — they’re often run by close associates of candidates or party insiders. The groups find plenty of creative ways to work closely with the people they support

These groups are allowed because the Supreme Court considers giving money to political causes a form of speech, which is protected by the First Amendment. The court has loosened the rules on political donations over the years and basically scrapped any limits on those outside groups altogether in 2010.