"Donald Trump has narrowed the gap on the Democratic front-runner: In a theoretical match-up, 43 per cent of registered voters would support Clinton and 37 per cent would support Trump," NBC News/Survey Monkey said in its weekly election tracking poll.
While Trump is six points behind Clinton, Senator Ted Cruz from Texas is 14 points behind the former Secretary of State in a potential November matchup between the two, the poll said.
The crucial primary was underway in Indiana. Trump was leading Cruz by a double digit margin in opinion polls ahead of the primary.
A victory in Indiana which has 57 delegates at stake would bring Trump within striking distance of bagging the Republican party nomination.
Compared to few weeks ago, Trump has gained ground in the match-up against Clinton, whereas Cruz has lost momentum, suggesting that perhaps voters are supporting Trump as the Republican Party's potential nominee more than they have in the past, the survey said.