COMMENT

After two years of Donald Trump and a divisive campaign, the US Midterm elections shape as an unofficial referendum on the US President.

He isn't on the ballot. The Midterms are largely about congressional seats and governorships. But historically in Midterms, the president's party is on the back foot as voters take the opportunity to protest against the incumbent's rule.

In this case, the Republican Party controls the executive and legislative branches of government - the presidency, the Senate and the House of Representatives. The situation is ripe for at least one limb to fall, with the House the most likely.

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In the House, the Republicans hold 235 seats to the Democrats' 193 with 218 needed for control. All 435 seats in the House are up for election, so the Democrats need 23. The Guardian reports that gains of 23-plus have only occurred five times in the last 50 years.

Conventional wisdom, based on polling, has it that the Democrats will take the House and the Republicans will hold onto the Senate, where they have a 51 to 49 advantage. In the Senate, 35 out of 100 seats are on the line.

But after the 2016 presidential election, no one is betting too hard on conventional wisdom and polling.

There are a number of unknowns and questions.

Will Trump in the end help or hurt his party? Is he more of a motivation for the Republicans or Democrats? What impact will independents play in deciding which of the major parties has the most to celebrate? Which issues and factors will prove decisive: Trump's performance; healthcare; immigration; jobs and the economy; taxes; regulations or the environment? What would it mean should Trump succeed after such a hugely negative, polarising campaign?

And what constitutes a good result?

For Republicans that would surely mean avoiding a House loss. The party would swagger towards the 2020 presidential election. Retaining overall control would mean being able fend off possible impeachment proceedings and the inevitable investigation a Democratic-controlled House would issue into Trump's election win.

Should a 'Blue Wave' wash away Republican control of Congress, Democrats would be able to set a legislative agenda for the next two years, although Trump could wield veto power. The Democrats could reject any new cabinet or Supreme Court appointments and go after Trump's tax returns.

For the Democrats it is boom or (nearly) bust: A shot of new confidence and power or a shattering setback await.

Apart from the big picture, there are interesting high-profile races and candidates. New Democratic star Beto O'Rourke is trying to unseat Texas senator Ted Cruz. African-Americans Stacey Abrams and Andrew Gillum hope to become governors. Dozens of former military veterans are running for office, including Republican John James who is hoping to become the first African-American from Michigan elected to the Senate.

There are a range of outcomes. Republicans have some advantages that could make it hard for Democrats to translate enthusiasm into concrete success. And yet there are other signs that a Blue Wave is a possibility.

THE NITTY GRITTY

Republicans

• Where the voters are.

Democratic voters are largely concentrated in cities and suburban areas, Republicans are largely in rural areas. But the electoral system is lopsided and unrepresentive with each state having two senators regardless of population. So a small rural state has as many senators as a major, urban-based, state. That blunts the Democrat population advantage. The special election in conservative Alabama showed that Democrats could be competitive in a red state. But home-ground advantage could matter in close races. Trump and his party have used the Supreme Court nomination and migrant caravan to drum up support on the right. We will see how successful that strategy was.

• The Democrats face a very difficult task in the Senate.
Eleven of their own seats - Missouri, Florida, Ohio, Virginia, West Virginia, Pennsylvania, Indiana, Wisconsin, Michigan, North Dakota and Montana - are tight races, according to the Washington Post. Only nine Republican seats are up for election this time, including six won easily by Trump, meaning the Democrats have to retain 26 of their own just to have a chance of tilting at three top targets: Tennessee, Arizona and Nevada. FiveThirtyEight puts the Democrats' chances of winning a majority in the Senate at 15 per cent.

• The economy remains steady.
Normally that's an aid to the governing party. The economic recovery that began under President Barack Obama has continued under Trump. The Republican has created 23,000 fewer jobs in his first 21 months than the final 21 months of Obama's presidency. The strong economy didn't help Hillary Clinton's argument in 2016 that voters should stick to the Democratic path, but there was also fatigue - the party had been in power for two terms.

• Traditional Republican voters.
The Obama coalition consisted of liberals, young voters, African-Americans and Hispanics. While Clinton drew enough of these groups to win the popular vote, Trump was able to ultimately dig out an Electoral College win thanks to working-class white voters in three swing states. The problem for Democrats is getting enough of their people to the polls. Midterm turnout is usually lower than for presidential elections and young voters make up a small percentage of the total. Republican voters tend to be older and more reliable about turning out. Core Trump supporters, including evangelicals, could simply be more organised and motivated.

Former President Barack Obama campaigned for Democratic governor candidate Andrew Gillum in Miami, Florida. Photo / AP
Former President Barack Obama campaigned for Democratic governor candidate Andrew Gillum in Miami, Florida. Photo / AP

Democrats

• The House map.

Republicans are defending 25 House seats that Clinton won and others that Trump won only narrowly. Politico says that around 100 Republican-held House seats are shaky. FiveThirtyEight puts the Democrats' chances of winning a House majority at between 78 and 85 per cent.

• The issues.
Healthcare has been a major source of motivation for Democrats under Trump. It's an emotional issue and it draws in independent voters.

• The generic ballot.
A traditional indicator of how it could all pan out is the question 'who do you prefer, Democrat or Republican?' The Democrats have been consistently leading this by about 8 to 10 points.

• Fewer incumbents to battle.
Dozens of Republicans - 39 in total - chose to retire rather than fight re-election, making their seats more competitive. Incumbents have voter loyalty, name-recognition and donor streams.

• Motivation.
Polls suggest more Democrats than Republicans are keen to vote. The Texas Tribune reported that turnout in early voting in areas of the state where 78 per cent of registered voters live has surpassed the entire turnout of the 2014 Midterms. Turnout among early voters aged 18 to 29 is up markedly - from a low base - across a number of states. These could be good signs for Democrats.

• The gender question.
High numbers of women are running for office as Democrats this time and women show out as far more anti-Trump in polling than men. This could be a key factor.

WHEN WILL WE KNOW?

Many contested House seats are in eastern states, where voting will be over by 1pm Wednesday NZT. It's possible that a verdict on the House majority could be clear by late afternoon. A longer wait is possible since there are competitive House races on the West coast. It could take days to get a result for the Senate because of mail voting.