In the latest charge to roil the tense Massachusetts Senate race, Democratic candidate Elizabeth Warren was accused this week of potentially practicing law in Massachusetts without a license.

Cornell law professor and legal blogger William Jacobsen made the claim on his Legal Insurrection website. He said Warren's legal practice is listed as the Bay State but no records show her having a license to practice law there.

Jacobsen says he found no law license under Warren’s last name or her maiden name, Herring, during an exhaustive search of state records. Jacobsen said he also confirmed with a clerk at the Massachusetts Board of Bar Overseers that Warren has never applied for a license.

Even so, it remains unclear whether Warren has violated any laws.

Issues about Warren’s legal practices began surfacing last week in her first debate with Republican Sen. Scott Brown – one that punctuated what has become a hard-hitting, razor-close race. An average of polls by the RealClearPolitics website has Warren leading by less than 2 percentage points.

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Brown -- who on Thursday repeatedly referred to Warren as "professor" during the candidates’ first debate -- reminded viewers that Warren in 2009 represented Travelers Insurance Co. before the Supreme Court in an asbestos-related case.

Warren had to have had an active state bar license when applying to try cases before the high court, according to the Supreme Court. So the more questionable issue appears to be whether Warren practiced state-level law from her Harvard office without a Massachusetts license.

Warren was paid $212,000 for three years of work in a case that involved thousands of asbestos victims and was “notable because Warren, who has gained fame for defending consumers against big business, was in this case working on behalf of a big business,” The Boston Globe wrote.

In addition, Warren, as a Harvard Law professor in 1995, attempted to help the LTV steel conglomerate avoid paying $140 million in benefits to retired coal miners, The Boston Herald reported Tuesday. Warren wrote a petition to the Supreme Court challenging a lower court ruling for the miners. The high court never took up the case, but Warren was paid roughly $10,000, according to documents obtained by the newspaper.

The Warren campaign had not responded to requests to verify whether the candidate has a license in Massachusetts or elsewhere.

Jacobsen writes that Warren listed in a 2008 resume that she has a license to practice law in Texas and New Jersey. Texas records show the license is inactive, but doesn’t indicate when it became inactive. And Warren reportedly cancelled her New Jersey license on Sept. 11 of this year.

Jacobsen also said he must make a written request in New Jersey to learn whether Warren's license was active during the Travelers case because the computer records show only the current status.

"It is unclear whether during the years she represented Travelers and others whether Warren was actively licensed anywhere," Jacobsen wrote in his blog.

The questions about Warren’s legal licenses follow repeated ones about whether she exaggerated her background in using her Native American ancestry to get law professor jobs at Harvard and the University of Pennsylvania.

A genealogist earlier this year claimed Warren is 1/32 Native American, though the documentation for that claim was later called into question.

Brown posted an ad Monday that revisits the questions about whether Warren used the Native American status for professional gain.

On Tuesday, Warren released a new, 30-second ad countering the attacks.

“I never asked for and never got any benefit because of my heritage,” Warren says in the ad. “The people who hired me said they didn’t even know about it. Scott Brown can keep attacking my family, but I’m going to keep fighting for yours.”