US President Donald Trump faced a test of his political influence on Tuesday in a special congressional election in Ohio that became a referendum on his leadership and a last chance to gauge Democratic strength ahead of November's midterm elections.

The Ohio race for a US House of Representatives seat headlines a day of voting across the country that also features primaries in four other states. Other key contests include a Democratic battle for governor in Michigan between a progressive and a mainstream candidate and a high-profile conservative challenge to the incumbent Republican governor in Kansas.

Trump charged into the Ohio race, the final special election before November. He visited the reliably Republican 12th Congressional District over the weekend to try to head off an upset after polls showed a tightening battle between Republican Troy Balderson and Democrat Danny O'Connor.

(With 91 percent of precincts reporting at 9:50 pm EDT Tuesday, the race was practically even, according to The New York Times.)

The district, split between suburban Columbus and rural areas, has been represented by a Republican since the early 1980s. Trump carried it by 11 percentage points in the 2016 presidential race.

But O'Connor has wiped out most or all of Balderson's lead in opinion polls, and a Democratic win would set off alarm bells for Republicans already worried by a series of strong performances in special elections by Democrats in the Trump era.

"A month ago this race was flying under the radar, but it has become nationalized and become more of a referendum on Trump, which really generates Democratic enthusiasm," said Patrick Murray, director of the Monmouth University Polling Institute in New Jersey.

A Monmouth poll last week showed the race was essentially a dead heat, down from a double-digit advantage for Balderson a month ago. In addition to more Democrats expressing high interest in the race, Murray said the shift was driven by independents unhappy with the status quo under Trump.

Trump again pushed Balderson in an early morning tweet before voting began.

Reuters