The time to start tuning in, the races to look out for, the chances of a Democratic ‘blue wave’, and why today’s result could completely alter the course of American politics.

READ OUR LIVE BLOG: Some important updates, and some unimportant updates, from America’s big day of democracy.

It’s been a long, long time coming. Two years almost to the day since Donald Trump was elected, shocking pundits and utterly ruining what had been a very pleasant Auckland spring day, today’s midterms finally offer an opportunity to put a check on the Ronsealed nightmare’s worst policies and abuses of power.

But why do the midterms matter, exactly?

Because right now the Republicans enjoy pretty much total control of the federal government. There’s Trump in the White House, of course, but Republicans also have majorities in the House of Representatives (“the House”) and the Senate. And because Trumpism – the racism, the Islamophobia, the misogyny, the attacks on the press – has proven so popular with the Republican base, a vanishingly small fraction of the president’s party has been willing to challenge him or his policies. Even worse, the subset of non-craven Republicans is getting tinier all the time: the Senate Republicans who once were willing to criticise Trump include Jeff Flake (now retired), Bob Corker (retired) and John McCain (dead). So today’s elections really are the last chance for a brake to be applied to Trump’s agenda before the presidential election in 2020.

Hang on, remind me what’s the difference between the House and the Senate?

The House is the lower chamber of Congress, made up of representatives from districts across the nation. Districts are apportioned by population, which means a state like California has far more House members than, say, minuscule New Hampshire. House terms are for two years only, so all 435 seats are being contested today. The Senate is a bit more electorally gnarly. Each US state has two senators, for a total of 100 senators, each serving a six-year term. Senate elections are staggered; around a third of the Senate is up for re-election today.

While both chambers of Congress can introduce legislation or launch investigations into, I dunno, collusion with Russia or the president’s tax returns, they also have some unique roles. The Senate is responsible for confirming federal judges – like Supreme Court Justice Brett Kavanaugh, plus 83 other lifetime judicial appointments so far this term, most of them hardcore conservatives often manifestly unqualified for the job.

As for the House’s special responsibilities, ever heard of a little thing called impeachment?

Oh yeah, that.

But hold your horses: while the House can instigate impeachment proceedings against the president – the first step in getting him booted out of office – the Senate is where he’d actually be tried, 12 Angry Men style.

OK, so the Democrats need to win both the House and the Senate. Easy.

You poor sweet child, if only it were so. While the surge in enthusiasm for the Democrats – the so-called blue wave – means a Democratic-controlled House is easily within reach (the uber-nerdy forecasting site FiveThirtyEight puts it at a 7 in 8 probability), the chances of the Senate flipping are much, much lower (just 1 in 5). The reason? It’s just dumb luck: of the 33 seats up for grabs this election, Democrats currently hold 23, including 10 in states won by Trump in 2016, some by huge margins. In order to win the Senate, they’d need to defend all those seats and win two more. Again according to FiveThirtyEight, this is the most unfavorable map that any party has ever faced in any Senate election.

But humour me – what’s the Democrats’ path to victory in the Senate?

OK, so let’s assume the Democrats manage to hold those 10 vulnerable seats (exceedingly unlikely given that Democrats Claire McCaskill in Missouri, Joe Donnelly in Indiana and Heidi Heitkamp in North Dakota are all in toss-up races). They’d still need to pick up two more seats. Their best shots are probably in Arizona, where Kirsten Sinema is polling neck-and-neck with Republican Martha McSally; Nevada, where incumbent Dean Heller (R) is in a tight contest with Jacky Rosen (D); and Texas, where one of America’s most charismatic politicians is attempting to overcome one of its most loathed. The battle between Robert ‘Beto’ O’Rourke and Ted ‘Zodiac Killer’ Cruz has captured the public’s imagination thanks to O’Rourke’s inspirational speeches, love of skate-boarding and Kennedy-esque good looks, but real talk: an upset victory in this deeply conservative state is probably not going to happen.

Anything else to look out for?

Are you a fan of those old comics where Nazis get knocked out by superheroes? Then you’ll love Iowa’s 4th district, where actual white supremacist Steve King could finally be shown the door by ex-professional baseball player JD Scholten.

And keep an eye on the governors’ races: they don’t tend to get the coverage of the nationwide contests, but they can be highly consequential, not only for the residents of those states but for the outcome of future national elections. That’s because governors exercise huge control over access to the vote: when someone new moves into the governor’s mansion everything from the number of polling places in the state to who actually has the right to vote there can be up for grabs. Two governors’ races to look out for are Georgia, where Democrat and long time voting rights activist Stacey Abrams is facing Republican and voting suppression enthusiast Brian Kemp; and Florida, which sees flawless Democratic debater Andrew Gillum go up against dog-whistling racist Ron DeSantis, the man responsible for what might be the most skin-crawling ad of the 2018 campaign.

Talking of Florida, today’s results will also include the outcome of a statewide referendum to restore the franchise to some 1.5 million ex-felons who currently have their voting rights stripped for life once they’re convicted. Overturning the law could help transform Florida’s ‘purple’ electorate – neither solidly Republican or Democrat – into one that’s solidly blue.

Right then. When should I start tuning in? And what signs should I be looking out for from the early results?

The first polls to close are at noon NZ time in Kentucky and Indiana; not long after we should get initial results in bellwether districts Kentucky 6th and Indiana 9th. Democratic wins in both, but especially the ‘lean Republican’ Indiana race, would be good signs of an oncoming blue wave. Between 1pm and 2pm expect the Florida and Georgia governor’s races to be called, and by 2pm results will be coming in thick and fast. Keep one eye on Twitter and the other on The Spinoff’s live blog, which we’ll be updating all day with election results and general US political weirdness.