The poll was conducted by Quinnipiac University, which surveyed 761 likely Iowa Democratic caucus participants from June 20-29, with a margin of error of 3.6 percentage points.
Sanders still has a long way to go to catch up to Clinton. However, Thursday's poll shows he has more than doubled his support from a May 7 Quinnipiac survey that showed him polling at only 15 percent to Clinton's 60 percent among caucus-goers. Thursday's poll shows the Vermont progressive having particular appeal among participants who call themselves very liberal, who chose him 47-43 percent over Clinton.
The other possible Democratic contenders remain stuck in the single-digits, with 7 percent of likely caucus-goers favoring Vice President Joe Biden (who has not declared a presidential run), 3 percent choosing former Maryland Gov. Martin O'Malley, 1 percent supporting former Virginia Sen. Jim Webb (who is expected to announce his candidacy in the days to come) and statistically 0 percent behind former Rhode Island Gov. Lincoln Chafee.