RICHMOND, Va.  As one of the keynote speakers here Friday at a state convention billed as the largest Tea Party event ever, Virginia Thomas gave the throng of more than 2,000 activists a full-throated call to arms for conservative principles.

For three decades, Mrs. Thomas has been a familiar figure among conservative activists in Washington  since before she met her husband of 23 years, Justice Clarence Thomas of the Supreme Court. But this year she has emerged in her most politically prominent role yet: Mrs. Thomas is the founder and head of a new nonprofit group, Liberty Central, dedicated to opposing what she characterizes as the leftist “tyranny” of President Obama and Democrats in Congress and to “protecting the core founding principles” of the nation.

It is the most partisan role ever for a spouse of a justice on the nation’s highest court, and Mrs. Thomas is just getting started. “Liberty Central will be bigger than the Tea Party movement,” she told Fox News in April, at a Tea Party rally in Atlanta.

But to some people who study judicial ethics, Mrs. Thomas’s activism is raising knotty questions, in particular about her acceptance of large, unidentified contributions for Liberty Central. She began the group in late 2009 with two gifts of $500,000 and $50,000, and because it is a 501(c)(4) nonprofit group, named for the applicable section of the federal tax code, she does not have to publicly disclose any contributors. Such tax-exempt groups are supposed to make sure that less than half of their activities are political.