The latest Quinnipiac University National Polls reported that Trump still leads the Republican presidential pack among party's voters nationwide, with 31 per cent votes, followed by Senator Ted Cruz of Texas with 22 per cent and Senator Marco Rubio of Florida with 19 per cent.
Neurosurgeon Dr Ben Carson has six per cent votes, with 9 per cent undecided and no other candidate above 3 per cent.
This compares to a 61 - 30 per cent Clinton lead in a December 22 survey by the independent Quinnipiac University Poll.
The university said Sanders and Rubio are the strongest candidates in general election matchups.
"If former New York City Mayor Michael Bloomberg runs, he hurts Sanders more than he hurts any of the top Republican contenders," it said.
Quinnipiac said among Republicans, 30 per cent say they "would definitely not support" Trump, while 15 per cent say no to Cruz and seven percent say no to Rubio.
The poll surveyed 484 Democrats and has a margin of error of 4.5 per cent.
"Democrats nationwide are feeling the Bern as Bernie Sanders closes a 31-point gap to tie Secretary Hillary Clinton," said Tim Malloy, assistant director of the Quinnipiac University Poll.
"And despite the Iowa setback, Donald Trump is way ahead of his GOP opponents. But that's not the whole story nine months before Election Day," he said.
