Explainer: What are the US midterms and why are they a big deal?

Explainer - American voters will soon get one more chance to cast their verdict on US President Donald Trump - quite possibly for the final time.

The US midterm elections are coming up on 3 November (expect results to come in after midday 4 November NZ time), and millions of dollars are being spent as candidates sprint around the country seeking votes. Early voting has already begun in many states.

But what exactly is being voted on? Here's what to look out for, and what Trump may face after results come in.

What are the midterms anyway?

They're a key part of the American political process where, halfway through a president's four-year term, voters get to pick their local congressional representatives and many senators and governors.

All 435 members of the House of Representatives and 35 of the 100 Senate seats are up for election this year, as well as 36 state governors.

The midterms are usually seen as a referendum on how a president's term is going and determine whether the US Congress is led by the Democrat or Republican party.

RNZ's Tim Watkin, executive producer of podcasts and series, is a keen observer of the American political scene.

The midterms are the biggest deal for the final two years of Trump's presidency, he said.

"It will change the balance of power in America."

If Trump's Republican party loses badly, "It will give Democrats the power to question Trump in a way he's never been questioned this term," Watkin said.

Explain to me again how America's Congress works?

We can try.

Instead of a Parliament, they've got Congress, which consists of two separate chambers - the House of Representatives and the "upper house", the Senate. Bills must pass both houses to become law.

The House has 435 members who are divided proportionally among the 50 states based on their population (so bustling California has the most reps, 52, while wide-open Wyoming has only one).

The Senate has 100 members and every state, regardless of size, has just two senators. The US Vice-President acts as a 101st member to break any 50-50 ties in the Senate.

The US Senate is considered more prestigious and powerful. Representatives only serve two-year terms, while senators serve six-year terms.

The current makeup of the House and Senate is very tight - there's a single seat majority in the House and six seats in the Senate. They've both been under Republican control since the 2024 election.

How big a deal is this for Trump?

Trump has operated the last two years under a Republican House and Senate and broadly exercised executive power with little pushback from fellow Republicans. That could well change after 4 November.

During his first term, when Democrats ran the House of Representatives, Trump was impeached twice, but both times the Republican-controlled Senate failed to convict him, which would have forcibly removed him from office.

If Democrats control both houses of Congress, look for an awful lot more talk about investigating alleged corruption, and perhaps even another go at impeachment.

While Trump has made lots of allusions to seeking a third term - which is illegal under the US Constitution - the reality is that an 82-year-old Trump probably won't be running again in 2028.

"It's Trump's last election," Watkin said.

"Once the midterm's done, especially if he loses badly, the next two years Republicans are going to be running a mile from him, and he's going to be standing there going, 'But I'm Donald Trump'."

Is history on Trump's side?

Not really.

Whether or not one supports Trump's policies, history is against him.

Over the last 80 years, the president's party has lost seats in the midterms 18 times out of 20.

"A sitting president gets held to account for not achieving the promises they made a year and a half ago," Watkin said.

It's almost inevitable that any US president, by the last few years of a second term, starts to enter "lame duck" status. Trump, who has defied a lot of political gravity in his career, is racing into that timeframe.

The current New York Times average of several political polls finds Trump's approval rating sits around 37 percent.

Watkin said that a faltering economy and the impact of the Iran war have hit many Trump supporters hard.

"Those men who have been his base are now pissed off because they're now paying [US] $6 for diesel in their trucks.

"He was supposed to be the businessman."

Current predictions by the influential nonpartisan Cook Political Report and others foresee the Democrats likely to take back control of the House of Representatives, while the Senate will be a closer call.

"They've given up in the House," Watkin said. "The idea that they could hold on to majorities is gone. The Senate's the thing that people need to watch."

And of course, nobody really knows how Trump will react if his party loses badly at the midterms.

"-he will claim corruption, right?" Watkin said.

What are the big races to watch out for?

Despite all 435 seats in the House of Representatives being up for election, only a handful are truly considered competitive. The New York Times and Cook Political Report currently list just 21 seats as toss-ups that could go either way, and another 21 as leaning somewhat towards Republicans or Democrats.

Because the margin of the current majority is so slim in the House, it would take only a handful of seats to flip to give Democrats control.

The Senate currently has a 53-47 Republican majority (two independent senators caucus with Democrats), and will be harder to flip. Democrats need to win 51 seats to achieve a majority as Vice President JD Vance can cast votes to break 50-50 ties in the current House.

The Senate, with its smaller membership, longer terms and more powerful legislative powers, is key. If Democrats capture the House and Senate, Republican Trump will lose any leverage to get his legislative priorities passed. CNN has ranked 10 seats that may flip in 2026.

There are several big Senate races to watch:

Texas - Democrat James Talarico is facing Republican Attorney General Ken Paxton. Normally Texas is deep red, but Paxton has been dogged by a series of scandals including a fraud indictment and impeachment. Polls show Talarico doing better than Democrats usually do there, but Republicans are pouring money into what will end up one of the most expensive Senate races of all time.

Maine - Democrat nominee Graham Platner's campaign imploded and replacement Troy Jackson hasn't quite gone so viral, but incumbent Republican Susan Collins has been in office since 1997 and is in danger due to the Trump backlash.

Ohio - A popular former senator, Democrat Sherrod Brown, is running to get back into office against Republican Jon Husted, who replaced JD Vance when he became vice president.

Michigan - One of the most combative races is between Republican Mike Rogers and progressive Democrat Abdul El-Sayed, who could become the first Muslim US senator.

Alaska - Remote Alaska would seem as Republican as they come but former Democrat Congresswoman Mary Peltola, the first Alaska Native elected to Congress, is running a close campaign against Republican Dan Sullivan.

Other important races are in North Carolina, Kansas, New Hampshire and Iowa. Many of the tight races are in states Trump won by a wide margin in 2024.

So are the Republicans doomed?

Well, we live in unprecedented times, and few political observers would've ever predicted most of what's happened in America in the past 10 years. America's deep political polarisation isn't likely to be resolved no matter what happens at the midterms.

Two big things still work in the Republicans' favour - the controversial redistricting of multiple states' congressional districts, which has seen both Republican and Democrat-controlled states trying to rejig electorates to ensure their party gets more votes. Republicans are generally seen as having created more possible seats for their party.

The second is money, which is the petrol running the US political engine. At least US$10 billion is believed to have been spent so far on advertising, by political parties as well as by big-pocketed donors such as the AI industry and Elon Musk.

Republicans are believed to still have more than US$1 billion to spend, and will pour money into places such as Texas' Senate race.

This year's election will break records, the nonprofit Brennan Center For Justice told The Washington Post recently.

"Having tons of money doesn't guarantee victory," US government ethics expert Meredith McGehee told the Post. "But the contrary is true - not having 'enough' does guarantee defeat. And no one ever really knows what 'enough' is."

And in case you're already dreading the endless carnival of the 2028 US presidential election, bad news - the race to succeed Trump will pretty much begin the day after the midterms. Expect candidates from both parties to start announcing their plans by early 2027.

Once again, when are those midterms?

They'll be taking place on Wednesday, 4 November NZ time, with results expected to start coming in early afternoon, and RNZ will be here to live-blog the fun as a warm-up before our own general election just a few days later.