Hillary Clinton may be leading Donald Trump in the polls, but there's another predictor of presidential elections that should have her scared.

According to two Halloween costume retailers, Trump masks have been outselling Clinton masks across the country.

Halloween mask sales have long been a predictor of the presidential election. One index shows that the candidate with the highest mask sales has won the election since 1980.

Retailer Spirit Halloween says their records have shown the same pattern going back to 1996.

Two Halloween retailers show Donald Trump masks are outselling Hillary Clinton masks. Above, people wearing masks of the candidates in New York City last Halloween

Data from Savers/Value Village shows that Trump masks are currently outselling Clinton masks by seven percentage points. Spirit Halloween shows a similar lead, with Trump 55 per cent to Clinton's 45 per cent.

However, experts point out that this year's election is different in that people aren't necessarily dressing up as the candidates as a form of flattery.

Kim Hermann, who owns Novelties Unlimited in Ghent, told the Virginian Pilot that this election is 'so contentious, most people buying candidate costumes are doing it to be negative. They’re planning to suggest something other than presidential behavior.'

'Oh, it’s definitely all about mocking them,' Chloe Nelson, a manager at Spirit Halloween added.

Backing up that is a survey of customers that Spirit has conducted this year.

When asked when they were dressing up as either candidate, the top response (at 39 per cent) was that they thought the mask was funny.

The poll also showed that there are twice as many people dressing up as Trump to mock him than people who are doing the same thing with Clinton.