The results of this year’s midterm elections will be enormously important — not just in shaping the future of Donald Trump’s presidency, but in shaping the American political landscape for a great many years to come.

A sprawling series of contests for a plethora of offices in different states will be on the ballot in November. There’s the whole House of Representatives, a third of the Senate, 36 governorships, and many state legislature seats. That’s so many races that only the most obsessive political observers could possible keep track of them all.

Naturally, there will be a whole lot of horse race coverage this year, over whether Democrats or Republicans are expected to do well or poorly. That’s all well and good.

But to understand why the question of who wins is so important — really, to understand why the midterms matter so much — it’s worth taking a step back and looking at the big picture of what’s at stake.

Depending on how well Democrats do, the party could kill the Republican legislative agenda in Congress, gain new powers to investigate the Trump administration, get the ability to block Trump’s nominees from being confirmed, pass new liberal state laws in many parts of the country, and win many offices with power over the 2021 redistricting process.

But if Democrats do poorly, they could feel the consequences of their failure for a generation.

That is to say: If Republicans hold Congress, they could revive their legislative agenda and have another shot at passing sweeping new laws. Accordingly, Democrats wouldn’t be able to issue subpoenas or block Trump nominees — potentially giving the GOP an opportunity to tip the Supreme Court further in their favor.

And if Republicans romp in state elections, they’ll be very well positioned to cement many gerrymandered district maps for their party for yet another decade — all the way through 2030.

So the midterms matter, quite a lot. Here are the main reasons why.

1) The Republican legislative agenda would be dead if either house of Congress flips

Republicans celebrate the passage of the tax bill at the White House in December.

Jabin Botsford/The Washington Post via Getty

First off, if Republicans lose control of either the House of Representatives or the Senate in 2018, they’ll lose their ability to send new bills to President Donald Trump’s desk with their party’s votes alone.

That means, basically, that the conservative legislative agenda would be dead.

Practically, the Senate filibuster rule already means 60 votes are required to advance most bills. And since the GOP only controlled 52 Senate seats, the party was far away from that threshold. So for the vast majority of votes on legislation (say, on funding the government), some Democratic support is already necessary.

But in a limited number of instances each year, Congress can use the special budget reconciliation process, which requires only a simple majority vote, to pass a new law. And Republicans aggressively used this process to try to enact their top two agenda items this year on a party-line basis — Obamacare repeal (which ended in failure) and their tax bill (which succeeded).

So if the GOP holds on to both chambers of Congress in 2018, the party could very well make another attempt at repealing Obamacare through reconciliation. (They were only one vote short of passing something through the Senate last time.) Some conservatives have discussed trying to tackle cuts to welfare, food stamps, Medicare, or Social Security with reconciliation as well. And more tax cuts are always a possibility with a Republican Congress — George W. Bush and Republicans used reconciliation to pass tax cuts through Congress in both 2001 and 2003.

Conversely, a Democratic takeover of either the House or the Senate would kill all those ambitions. The practical consequences would probably be: no Obamacare repeal; no major cuts to Medicare, Social Security, food stamps, or welfare; and no more big tax cuts for corporations and the wealthy.

Instead, we’d likely get gridlock. Even if Democrats won both chambers, they wouldn’t be able to enact their own new partisan priorities, so long as President Trump remains in office and can wield his veto pen.

So the real stake on this front are: Do conservatives get more chances to enact their dream laws in 2019 and 2020 — laws that could have consequences for decades to come — or do they get stopped in their tracks?

2) Many more investigations into Trump’s administration would ensue if either house of Congress flips

Currently, Republican Devin Nunes (center) chairs the House Intelligence Committee. Democrat Adam Schiff (left) would like to do so.

Tom Williams/CQ Roll Call

Winning control of the House or Senate wouldn’t just give Democrats veto power over new bills. It would also give them subpoena power — which would let them investigate the Trump administration far more aggressively.

House and Senate committees can send subpoenas for documents and can compel witnesses to come in and testify. However, those committees are controlled by the majority party.

So right now congressional Republicans are deciding whom to subpoena, and on which topics. (They sometimes grant Democratic requests, but it’s up to them to decide whether to do so.) And in general, the GOP is not all that motivated to dig too deeply into matters that could embarrass Trump or his administration.

But Democrats very much would be. If the party regained subpoena power, key members of Congress would likely investigate much more than Russia. They could take on Trump’s businesses (such as questions over influence peddling at the Trump Hotel and Mar-a-Lago), the sexual assault allegations against Trump, and all sorts of controversial policy choices or potential scandals in the administration.

And while there are already active investigations into the Trump campaign’s connections with Russia — both from special counsel Robert Mueller and from several congressional committees — if the committees were led instead by Democrats, they’d surely be far more aggressive in their inquiries on the topic.

Though congressional investigations sometimes do unearth crimes, they can also be enormously politically impactful even if they don’t. For instance, the House GOP’s aggressive, years-long investigations of the Obama administration’s handling of the 2012 Benghazi attacks didn’t find any wrongdoing on that front. But it did bring to light the topic of Hillary Clinton’s email use as secretary of state, which proved enormously damaging to her presidential campaign.

As to whether these investigations might lead to Trump’s impeachment, of course no one can say now. But the impeachment process starts in the House, so it’s certainly more likely to kick off if that chamber is controlled by the president’s enemies. (Still, it takes a two-thirds vote in the Senate to actually remove a president from office, so some Republicans would have to go along here too — meaning it would only happen if there were some genuinely remarkable revelations.)

3) Trump would be enormously constrained on judges and other nominees if the Senate flips

Trump and Supreme Court Justice Neil Gorsuch.

Ricky Carioti/The Washington Post via Getty

The battle for control of the Senate will have another enormous consequence: If Democrats retake the chamber, they’d get veto power over Trump’s nominees.

Unlike most legislation, Senate rules now let Trump get any nominee through the chamber with a simple majority — meaning he’s gotten a plethora of nominees confirmed without any Democratic support.

But under a Democrat-controlled Senate, that would no longer be possible. The much more consequential result would be that Chuck Schumer would decide on the calendar for considering and confirming nominees — which would let him bury many of Trump’s picks indefinitely.

This would be most important of all for the narrowly divided Supreme Court. Democrats have long feared that a vacancy in a liberal or swing seat could arise while Republicans controlled the presidency and Senate, because their nominee could serve for life and reshape American jurisprudence for decades.

But if Democrats retook the Senate, they could and likely would simply refuse to consider any Trump nominee for the seat — as Republicans did for Obama’s nominee Merrick Garland in 2016. Lifetime appointments to lower federal courts, something the Senate has been moving quickly on under Trump and McConnell, would also surely slow to a crawl.

Finally, Trump appointments to Cabinet positions and other executive branch positions would also have to win Democrats’ approval if the party controlled the Senate. This would mean not only that Democrats could block nominees they consider extreme or unqualified, but also that they could prevent Trump from appointing sycophants to key federal law enforcement posts.

4) State elections could lead to new Democratic majorities, and new liberal legislation

Phil Murphy’s win in New Jersey’s 2017 governor race will give Democrats full control over the state’s government.

DOMINICK REUTER/AFP/Getty

The Democratic Party’s performance in state elections over the past eight years has been positively dismal. As of mid-January of this year, Republicans will control 33 out of 50 governorships and at least 66 out of 99 state legislature chambers. The GOP even has veto-proof supermajorities in 17 state legislatures.

This dominance stems mostly from Republican landslides in the 2010 and 2014 midterm elections, which allowed the party to advance sweeping conservative legislation in many states across the country (and to draw district lines to help protect their majorities — more on that below).

But now a whole lot of those governorships and state legislature seats will be up for grabs again — giving Democrats the chance to make similar gains, or at least to make substantial improvement from where they are now.

Thirty-six governors’ races will be on the ballot this year (26 of those are currently in Republican hands).

At least half of state Senate seats will be on the ballot in 42 states this year. (The entire state Senate will be on the ballot in 15 of those states.)

Every state House seat will be on the ballot in all but a handful of states this year.

Thirty state attorney general offices will be on the ballot as well. (Another five state AGs will be chosen by the winners of either the governor or state legislature races this year.)

The more governorships and state legislatures the Democrats manage to win, the more opportunities there will be for new liberal legislation across the country. And if Democrats pick up many state attorney general offices, they’ll have many more people looking to file lawsuits against the Trump administration for policies they find legally dubious or controversial.

5) Finally, a huge chunk of the 2021 redistricting will actually be determined by this year’s elections

The 2010 census form.

Robert Daemmrich Photography Inc/Corbis via Getty Images

These midterms won’t just be consequential for the next few years. Their results will help define the battlegrounds of political combat all the way through the year 2030 — because of the impact they’ll have on the next round of redistricting.

The US Census is conducted every 10 years, with the next census scheduled to take place in 2020. New districts for US House and state legislative races will be based on the results and take effect ahead of the 2022 elections.

States’ redistricting processes vary; Justin Levitt of Loyola Law School has a good breakdown of how they work. But in most, the state legislature runs the process, with the governor usually getting veto power.

Last time around, that meant the GOP landslide of the 2010 midterms took place during a census year. The election installed all those new Republicans in state legislatures and governors’ mansions just in time for redistricting, giving them far more opportunities to gerrymander district boundaries to their liking.

The next redistricting won’t start until after the 2020 election. But crucially, many of the state politicians who will be in office for that redistricting will have been elected to four-year terms in 2018.

Indeed, in 34 states, the governor who will be in office for the next redistricting will be elected this year (two more were just elected in 2017). And in 30 states, half or more of state senators whose terms extend through the next redistricting will be elected this year. (Most state House members, meanwhile, serve two-year terms and will be elected in 2020.)

The upshot is that if Democrats want to reverse GOP gerrymanders — and perhaps get the opportunity to do some gerrymandering of their own — they can’t afford to sit around and wait for 2020. They need to make big gains in the states this year. The stakes are high, and the consequences will be felt for a very long while indeed.