Elizabeth Warren, the Democratic candidate in a pivotal U.S. Senate race in Massachusetts, admitted Monday that she never was licensed to practice law in Massachusetts even though she was representing clients out of her faculty office at Harvard University for years.

Warren, who already suffered setbacks in her effort to unseat Republican incumbent Sen. Scott Brown because of her claims of Native American heritage, said on a talk show on Boston's WTKK-FM that she had surrendered her law license in New Jersey but was never admitted to the bar in Massachusetts.

"I've been inactive in the bar for a very, very long time," Warren said. Her license lapsed in New Jersey because she could not continue to attend the legal classes needed to maintain it, she said.

The revelation raises questions about Warren's participation in several legal cases, including her representation of Travelers Insurance in an asbestos liability case that was finally decided by the U.S. Supreme Court in 2009. Warren was paid more than $200,000 in that case, the Boston Globe reported.

"Based on the way the law was written in Massachusetts, it appears she is not in compliance," William Jacobson, a Cornell Law School professor, told The Washington Examiner.

Jacobson was the first to uncover Warren's law license status and wrote about it on his blog, Legal Insurrection. The blog documents Warren's continuing law practice and the use of her Harvard office as her legal office, which also violates a Massachusetts law requiring anyone practicing law out of an office to have a state license.

Warren's press office did not respond to a request for comment from The Washington Examiner, but when Warren was asked by the radio hosts if she practiced law in Massachusetts, she said no.

The news came on the day Brown, her GOP opponent, stepped up his attacks on Warren's credibility.

Brown released an ad Monday focusing on Warren's claim in law directories that she was of Native American heritage. Warren said her family has long believed that they were part Cherokee, but insists she did not use the minority status to secure a job at Harvard.

Brown's ad -- entitled "Who knows?" -- ends with a reporter asking Warren, "Is there anything else that we don't know that is going to come out about you?"

Brown has demanded Warren release records proving she did not use her claim of minority status to advance her career.

The race between Brown and Warren is one of the most closely watched and hotly contested Senate races in the country. Brown won the seat in a heavily Democratic state and Democrats now hope to win it back in their effort to maintain narrow control of the Senate.

Polls show Warren and Brown in a close race, and Republican strategists said the latest flap over Warren's legal credentials could give Brown an edge.

"This could hurt her in a number of ways," said Todd Domke, a GOP strategist based near Boston. "It would fit into this pattern that Brown is drawing, that there are a lot of doubts about her character."

However, Robert Boatright, a political science professor at Clark University, said the attacks on Warren could backfire.

"It's an awful lot of negative stuff," Boatright said. "The danger for Brown is overkill."