Even in a year when the presidency isn't being decided, election day in the US is massive. There is voting for state legislatures, local councils, referendum initiatives, and more. But most of the interest is in three things: the two houses of Congress, and state governors. Let's take them in turn.
House of Representatives
The whole of the lower house is elected every two years. In 2016, the Republicans won a 47-seat majority, 241 to 194. The Democrats picked up one seat in a byelection (Americans call them "special elections") earlier this year, so they need another 23 to take control.
Because it's just a large number of single-member districts, you can look at the House simply on the basis of swing on the pendulum, as we would in Australia. Swings are never uniform, but deviations from uniformity will roughly cancel out. (There are no preferences, so minor party votes don't count; I factor them out.)
The big difference from Australia is that the boundaries are badly gerrymandered. The Republicans won their substantial majority with just 50.6% of the two-party vote, and to undo it the Democrats will need to win about 56%, or a swing of about 6.5%.
That's quite a task, considering how American incumbents typically entrench themselves. But the Republicans are saddled with a deeply unpopular president, and the Democrats are riding a wave of enthusiasm, particularly in the suburbs, that looks likely to propel them to a majority.
Nate Silver's FiveThirtyEight, the most respected prediction site, gives them an 87.5% chance. Those are good odds, but no certainty.