Washington — A top Senate aide responsible for much of the hard-nosed scrutiny of colleges and other nonprofit organizations in recent years has resigned.

Dean A. Zerbe, the senior counsel for Sen. Charles E. Grassley, the top Republican on the Senate Finance Committee, announced this morning that he would leave his post at the end of February to become national managing director of Alliantgroup, a Houston-based tax-consulting company.

Since 2001, Mr. Zerbe has been the engine driving the ambitious agenda on nonprofit issues pursued by Mr. Grassley, shaping many of the committee’s moves on tax policy related to universities.

Most recently, Mr. Zerbe has led the Iowa senator’s investigation of college endowments, pushing the biggest institutions to spend more of their assets every year.

He has also scrutinized the offshore hedge-fund investments many college endowments make, helped investigate how universities use tax-exempt bonds, and organized a hearing on rising tuition and higher-education tax incentives.

And he helped oversee the senator’s investigation of governance problems at American University, calling its Board of Trustees a “poster child for why review and reform are necessary” for governing boards of nonprofit organizations.

Mr. Grassley plans to replace his top investigator with someone who will continue the committee’s scrutiny of colleges and other nonprofit groups, said a lobbyist familiar with the senator’s plans.

But they will be big shoes to fill. “They’ll never have Dean’s energy,” said the lobbyist. “Once you were in his sights, you know, he was ruthless.” —Brad Wolverton