Asian shares stumbled and the US dollar on was on the defensive on Wednesday morning as signs that Donald Trump could be closing the gap with his rival Hillary Clinton spooked investors.

Anxiety in markets has deepened over a possible Trump victory given uncertainty about the Republican candidate's stance on several issues including foreign policy, trade relations and immigrants, while Clinton is viewed as a candidate of the status quo.

MSCI's broadest index of Asia-Pacific shares outside Japan dropped 0.4 percent while Japan's Nikkei fell 1.1 percent.

A Washington Post poll on Tuesday had the gap between the two candidates at just one per cent.

That pushed the US S&P 500 Index down to a four-month closing low on Tuesday. The CBOE volatility index, often seen as investors' fear gauge, briefly rose to a two-month high above 20 percent.

In the currency market, traders sold the dollar partly as they suspect Trump would prefer a weaker dollar given his protectionist stance on international trade.

On the campaign trail last night, Mrs Clinton launched a fierce attack on a heckler at a rally in Florida who shouted that her husband Bill Clinton was "a rapist".

The Democratic nominee slammed the "dark, divisive, dangerous vision" of Mr Trump after she was interrupted by the heckler during the rally, which did not start until way after 10pm local time.