Three weeks from the start of primary season in the US, Bernie Sanders has scored his first lead over frontrunner Hillary Clinton in the Iowa caucus, the first contest in the race for the Democratic presidential nomination.

The results of poll released on Tuesday by the independent Quinnipiac University found that the Vermont Senator is drawing 49 percent support among likely Democratic voters, compared with Clinton's 44 percent and former Maryland governor Martin O'Malley's 4 percent. The latest survey showed an abrupt reversal of fortunes for the top two Democratic candidates from a December 15 poll that had Clinton leading Sanders in Iowa 51-40.

Support for Sanders, a self-described democratic socialist, has surged since he announced his candidacy last May. The 74-year-old Sanders received an unprecedented 2.5 million individual financial contributions to his campaign last year. Despite the upward trend in public opinion polls for Sanders, doubts about his chances of winning the nomination over the former secretary of state have dogged his campaign. Sanders currently lags some 8.6 percentage points behind Clinton in a composite of Democratic nomination polls provided by Real Clear Politics.

"People keep saying 'I love Bernie, but he'll never win,'" Moumita Ahmed, a 25-year-old organizer for a number of Sanders grassroots groups, told VICE News in November. "But if those people actually came out to vote for Bernie, he'd have a chance."

The results of the poll Tuesday cemented some of those assumptions, indicating that 85 percent of Democrats still thought Clinton would have a good chance of winning the presidency, while only 68 percent of respondents said the same thing of Sanders.

'The playing field has changed.'

The Quinnipiac poll, which surveyed 492 likely Democratic Caucus participants, seems to suggest that Sanders is gaining ground in Iowa, which holds its vote on February 1. A majority of respondents — 51 percent — said Sanders could best handle the economy, compared to 39 percent who thought Clinton the better option. Sanders also led 51–32 percent when voters were asked about who could better address climate change. Women preferred Clinton 55-39 percent, while Sanders had the support of men 61-30 percent over Clinton.

"Iowa may well become Sen. Bernie Sanders' Field of Dreams," said Peter A. Brown, assistant director of the Quinnipiac University Poll, referencing the 1989 Kevin Costner film about an Iowa corn farmer that featured the mantra, "If you build it, he will come."

"After three months of Secretary Hillary Clinton holding an average 10-point lead among Iowa Democrats, the playing field has changed," Brown said.

According to another survey of likely voters released on Sunday by NBC News, the Wall Street Journal, and Marist, Clinton holds just a three-point lead over Sanders in Iowa, 48 percent to 45 percent. The same poll had Sanders leading Clinton 50 percent to 46 percent in New Hampshire, which holds the next primary after Iowa on February 9. Both leads were within the poll's margin of error of 4.8 percent.

The NBC/WSJ/Marist poll also had Sanders outperforming Clinton in hypothetical general-election matchups in New Hampshire and Iowa against the top Republican candidates. Sanders is projected to handily defeat Donald Trump, Ted Cruz, or Marco Rubio, while Clinton trails Trump by one percentage point, Cruz by four points, and Rubio by nine points. The poll said Sanders was outpolling Clinton mainly because of his "stronger performance with independent voters."

Brown said that the Democratic race lacked the "divisive tone" of the Republican contest.

"Iowa Democrats like both major candidates personally; they just like Sen. Sanders more," he said.