Democratic front-runner Hillary Clinton has a higher probability than her likely Republican rival Donald Trump of becoming the next US president, but the gap between the pair narrowed this week, according to the online political stock market PredictIt.

Clinton's probability on Friday was 61 per cent, down from 65 per cent seven days ago, according to the site, which allows users to wager small amounts of money on "yes" or "no" predictions of future events. The probability that Trump will win the November 8 election was 40 per cent, up from 34 per cent.

Trump's sweeping victory in this week's Indiana primary prompted his remaining Republicans rivals Ted Cruz and John Kasich to drop out of the race, cementing the businessman's status as the party's presumptive nominee.

He is now testing out themes to use against Clinton to persuade disgruntled Republicans to get behind his campaign. On Friday, he criticized her use of a private email server while she was US secretary of state.

PredictIt is jointly run by Washington political consultancy Aristotle and Victoria University in Wellington, New Zealand. All of its users are registered US voters.

As with polls, predictions markets like PredictIt do not always accurately forecast outcome. Ninety days ago, its users gave Trump a 20 percent probability of winning the presidency, illustrating how the celebrity businessman's momentum was underestimated.-Reuters