The 2018 midterm elections held on Tuesday, November 6, are the first under President Donald Trump, and the stakes for both parties are incredibly high. A fired up Democratic base threatens Republicans’ unified control of federal government — and the outcome could have lasting consequences for the remainder of President Trump’s presidency and his legacy.

If Democrats do well and flip one or both chambers of Congress, they could slow down Trump and the Republicans’ legislative agenda — or open up serious investigations into Trump, his administration, and his associates. But if the GOP retains control of both chambers, they’ll have a shot at introducing sweeping policy changes that could impact our nation for decades to come. So far, the Republican agenda has largely been limited to tax cuts, but they have a long list of conservative agenda items — perhaps even returning to repealing Obamacare.

Meanwhile, there are dozens of governors’ mansions and statehouses around the country up for grabs; the results help decide who gets to redraw congressional maps in 2020. Whoever starts winning in November could influence national politics for the next decade.

Here’s a look at everything you need to know to understand the 2018 midterms — from the policy stakes to which states and races are most important to watch.