The U.S. midterm elections don’t usually get a whole lot of attention outside of the country. But this year is different because whatever happens in November will be seen as a referendum on Donald Trump. Right now, Republicans have the majority in both the House of Representatives and the Senate. But control of Congress is up for grabs. If nothing changes it means Americans like what they have now and they want more of it. But if Democrats take control of either house there could be some major changes. For starters pretty much any Republican legislation would be dead on arrival and expect more investigations into allegations of corruption Russia and Trump’s businesses. So how do the midterms work? Let’s start with the House. Members serve a two-year term, so every seat is up for grabs. Right now Republicans have 236 out of 435 seats. Democrats have 193 and six are vacant. One reason Democrats are optimistic: more than 40 Republicans aren’t running again, including House Speaker Paul Ryan. Flipping the house is rare. In fact in the last 25 years, it’s only happened three times. In 1994 back when Bill Clinton was president Democrats lost the house. In 2006, it flipped back, partly because of anger with then-President George W Bush. In 2010, the Tea Party wave handed control back to the Republicans, and they have kept it ever since. What do those three flips have in common? Each favoured the party not in The White House. The Senate works differently. There are 100 senators, two for each state. Republicans have control of the Senate by a paper-thin margin. But it’s pretty likely they’ll still have control come November. Each senator serves a six-year term and elections are staggered – they happen every two years. So about a third of the Senate is up for re-election. This year, 35 seats are in play. 26 are held by Democrats or left-leaning independents. 9 are held by Republicans. So the basic math favours Republicans and 10 of those Democrats are running in states that Trump won and is still popular. So the Senate changing hands could be a long shot. But it’s not just Congress that could change. There are 36 governorships up for grabs and more than 6,000 members of state legislatures will be elected. A record number of women are running for office, most of them Democrats. So the results could be historic. Another thing to watch for in all of this is turnout. Traditionally more Republicans show up to vote for the midterms than Democrats. Opposition to Donald Trump could change that or not and then there are concerns about Russia interfering in the election. All of this while the tone is already nasty on both sides and the country is increasingly divided.