It comes after the New York real estate mogul has won three consecutive early contests and is on the cusp of a major, must-win primary battle for Cruz in Texas Tuesday.
Still, the poll shows a divided party. Almost half of GOP voters who don't support Trump say they probably or definitely will not support Trump in the general election.
Trump's support also remains widespread across every demographic group, including gender, ideology, age and even education level, where Trump had lagged behind among voters with a college degree.
Other recent national polling has shown Trump with a double-digit lead and support from about one-third of GOP voters.
Trump has been under pressure this weekend for declining to condemn a leader of the KKK. This poll was taken partially before and partially after last week's debate, during which Rubio and Cruz sparred with Trump in the most direct way yet.
Seven in 10 Republicans now believe Trump is most likely to win the party's nomination.