The NBC News/Survey Monkey Weekly Election Tracking Poll found Democratic presidential nominee Clinton with a mere four-point lead on Trump, 48 per cent to 44 per cent.
The same poll last week found Clinton with a bigger lead, winning 48 per cent of the voters to Trump's 42 per cent.
Clinton had a 10-point lead over Trump in the same poll five weeks ago.
When third-party candidates are factored in, Clinton's lead slips further, with the Democrat garnering 42 per cent of the vote, Trump 40 per cent, Libertarian Gary Johnson getting 11 per cent and Green Party candidate Jill Stein at found per cent.
The poll finds Trump surging among independent voters, with the mogul trailing Clinton by only two points among those voters, 38 per cent to 36 per cent.
Trump used to be down among independents by double digits, NBC reports. Voters who describe themselves as independent are thought to be crucial to winning the presidency.
Favourability ratings for the two main presidential candidates are similar (38 per cent have a favourable view of Trump, 39 per cent of Clinton).
And the candidates share a similar unfavourable rating (60 per cent view Trump unfavourably and 59 per cent have that view of Clinton).
This week's NBC News/SurveyMonkey online poll surveyed 16,220 registered voters and was taken between September 5 and 11.