The US midterm elections will be the first test of how Donald Trump's doing as president halfway into his four-year term.
So what exactly are these elections, and how do they work?
It's all about what's inside the US Capitol building, home to the House of Representatives with 435 members and the 100-seat Senate.
Republicans - Trump’s party - currently have the majority in both chambers.
All members of the House are up for election, and Democrats are talking up a blue wave taking over. They need just 24 seats to win control.
In the Senate, just 35 seats are up for grabs.
Of those, 26 are already held by Democrats, which is why many are tipping Republicans to retain their slim majority.
1 NEWS US correspondent Rebecca Wright is on the ground in Arizona.
But Democrats only need two for an upset, and to deal a massive blow to Trump. Though they also need to hold all their current seats, including some in states where Trump did well in 2016.
Traditionally, the president’s party does badly in the midterms, and Trump also has low poll ratings compared to other presidents at this point in their administration.
Control of the Senate means control of how the presidency will be policed, including possible impeachment proceedings, and means the key positions on the main committees. Without it, Trump’s hand would be considerably weaker.
A handful of states will be key.
Arizona in the Southwest is important. The Democrats have a real turnover shot here and, as a border state, the critical issue is immigration.
Nearby in Nevada, where it's 50/50 odds at the moment, both parties have been peppering this state as voter turnout will be crucial.
Down south in Georgia, race is key with claims of anti-black and minority voter suppression.
In two days, American voters will send a message to President Donald Trump. But what that message is remains to be seen, says our US correspondent Rebecca Wright.
If the Republicans win these key states, it’s a vote of confidence for Trump. If not, the race for the next presidential election in 2020 is well and truly on.
There are also state, local and governorship elections across the US tomorrow.
Everything will become clear on Wednesday afternoon and evening NZ time, and 1 NEWS Now will have live streaming and live coverage of the results as they come in from 2pm.
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