This is the first after the New York-based real estate mogul joined politics last summer from the crowded 17-person Republican race to the White House.
"This milestone is significant as the 2016 primary heads into its final few weeks of contests, as there has been intense speculation that Trump's support has a ceiling. Though his support has hovered in the high 40s since mid-March, the front-runner had yet to secure half of Republican voters," NBC News said.
On the other hand, the popularity of Trump's nearest Republican rival Texas Senator Ted Cruz, 45, has dropped slightly by two percentage points to 26 per cent. Ohio Governor John Kasich, 63, has 17 per cent support.
The NBC News/SurveyMonkey Weekly Election Tracking poll was conducted online April 18 through April 24 among a national sample of 10,707 adults aged 18 and over, including 9,405 who say they are registered to vote.
As many as 172 delegates are in fray for the five State East Coast primaries being held today in Maryland, Delaware, Pennsylvania, Connecticut and Rhode Island.