Key European markets were down by up to one percent at the close, while Wall Street reversed early declines to post timid gains approaching midday in New York.

But US market gains appeared less than solid, with Briefing.com analyst Patrick O'Hare saying "don't be surprised if today's action has a seesaw feel to it".

Most analysts expect the Democrats will win the House of Representatives, while the Republicans hold onto control of the Senate.

Conventional market wisdom says that equity prices should fall if the Republicans lose both Houses, and should rise if they retain both, dealers said.

Earlier Tuesday, Asian indices had mostly ended higher.

The dollar was under pressure against European currencies.

"Americans will cast their crucial votes in today's midterm elections, which for many will be a referendum on Trump's presidency," noted Rabobank analyst Jane Foley.