Democratic presidential hopeful Bernie Sanders has inched ahead of rival and presumptive nominee Hillary Clinton in the most recent poll in Iowa. The Vermont senator was the pick of 41 percent of likely Democratic caucus voters, while 40 percent chose Clinton, according to results of the Quinnipiac University poll released Thursday.

The race in Iowa has tightened as Clinton has waded through continued controversy surrounding her use of a private email server while she served as secretary of state under U.S. President Barack Obama. Vice President Joe Biden earned the third-largest share of support at 12 percent, despite the fact that he has yet to come to a decision about running for the 2016 nomination. Former Maryland Gov. Martin O'Malley earned the support of 3 percent of likely Iowa caucus voters. 

Sanders' lead is slight, falling well within the margin of error in the poll, but notable nonetheless. The climate in Iowa has shifted significantly through the course of the summer. A July Quinnipiac poll had Clinton earning 52 percent, Sanders at 33 percent and Biden at 7 percent. Sanders is also polling ahead of Clinton in New Hampshire, indicating the Vermont senator could be taking advantage of Clinton's continuing public troubles. 

"He is the candidate of the Democratic left, against his own party's bosses and their prized presidential candidate, Secretary Hillary Clinton," said Peter A. Brown, assistant director of the Quinnipiac poll, in a statement. "Sanders has seized the momentum by offering a message more in line with disproportionately liberal primary and caucus voters." 

Clinton Clinton and Sanders are both vying for labor's blessing during primary season. In this photo, Clinton partakes in a Q&A after discussing the Iran nuclear agreement at the Brookings Institution in Washington, Sept. 9, 2015. Photo: Reuters

While Sanders may have a shot at New Hampshire and Iowa, Clinton is still the heavy favorite on the national scene, leading about 47.8 percent to 23 percent, according to an average of polls from Real Clear Politics. Politico reported that people close to Clinton's campaign have warned that Sanders could take early caucusing states Iowa and New Hampshire before her "massive organizing infrastructure" allows her to win later states. 

Clinton apologized for the first time Tuesday concerning her use of a private email address as secretary of state instead of a government account, perhaps hoping to shake the continuing controversy.  "That was a mistake,” Clinton told ABC News, just one day after rejecting the need to apologize. “I'm sorry about that. I take responsibility. And I'm trying to be as transparent as I possibly can."

Both Clinton and Sanders were viewed favorably overall by Iowa democrats in the poll, however. Clinton was viewed favorably by 76 percent of those polled, Sanders by 78 percent. The Quinnipiac poll surveyed from 832 likely Democratic caucus participants from Aug. 27 - Sept. 8, with a margin of error of 3.4 percentage points.