The first polls began to close in parts of Indiana and Kentucky on Tuesday as Americans cast votes at the end of a divisive campaign to decide whether Donald Trump's fellow Republicans maintain their grip on the U.S. Congress.

The remainder of the polling locations in Indiana and Kentucky, and in half a dozen other states, will close in another hour. But it could be hours before the winners are determined in dozens of tight races that will decide control of the House of Representatives and Senate.

The first national elections since Trump captured the White House in a 2016 upset became a referendum on the polarizing president, and a test of whether Democrats can turn the energy of the liberal anti-Trump resistance into victories at the ballot box.

At stake are all 435 seats in the House of Representatives, 35 seats in the 100-member Senate, 36 governor's posts and seats in state legislatures across the country.

For almost two years, Trump's rule-breaking, sometimes chaotic administration has enjoyed a largely free hand from the twin Republican-controlled chambers, but the midterms could finally see his wings clipped.

According to nearly all pollsters, the Democrats have a good chance of winning the House, while the Republicans are likely to retain the Senate.

But with turnout a key unknown factor and pollsters still unsure about the effect of Trump's maverick style on voters, both parties admit that they may be in for nasty surprises.

After a campaign in which Trump was accused of race-baiting with repeated and unsubstantiated references to an "invasion" of undocumented immigrants bent on rape and murder, left-right divisions in America could not be deeper.