DENVER — The Democratic chairman of the Senate Finance Committee has thrown his support behind an investigation of allegations of lavish spending and lax oversight at a half-dozen "prosperity gospel" Christian ministries.

Sen. Max Baucus of Montana joined with Iowa Republican Sen. Charles Grassley in urging cooperation from four ministries. According to a statement from Grassley's office Wednesday, the four ministries have not responded to questions he posed in early November.

Follow-up letters the two senators have sent to the three ministries that have refused to answer Grassley's questions give a March 31 deadline for turning over the requested information. A fourth ministry has indicated it will cooperate.

Having Baucus on board is crucial for any potential bid to seek subpoenas forcing answers from uncooperative ministries. Grassley critics, including Creflo Dollar, one of the ministers not cooperating, have portrayed the senator as a renegade who has no real power because his party is in the minority.

The follow-up letters do not mention subpoenas specifically, but express hope that the requested information can be obtained "without resorting to compulsory process."

Some ministries have said Grassley's inquiries about private planes, oceanside mansions, board oversight and involvement in for-profit businesses should be handled by the Internal Revenue Service, but the latest letters cite the Finance Committee's jurisdiction measuring the effectiveness of tax-exempt policy, saying that role is distinct from the IRS job of enforcing existing law.

"This ought to clear up any misunderstanding about our interest and the committee's role," Grassley said. "We have an obligation to oversee how the tax laws are working for both tax-exempt organizations and taxpayers."

All the targeted ministries say they follow IRS rules governing churches.

So far only one ministry, led by St. Louis-area author and speaker Joyce Meyer, has "cooperated substantially" and turned over information, Grassley's office said.

Another, headed by Texas-based faith healer Benny Hinn, has indicated cooperation but so far has answered just five of 28 questions, Grassley's office said. Neither Meyer nor Hinn were among those receiving the follow-up letters.

The follow-up letters were sent to Dollar and Bishop Eddie Long, both of the Atlanta area, and Kenneth Copeland, whose ministry is based at Newark, Texas.

All three have pledged to fight what they portray as an attack on their religious freedom. Some have expressed worry that private information, including salaries and the donors' identities, would be made public.

The letters from Baucus and Grassley say the committee is willing to work with them to protect confidential information.

A fourth letter went to Paula and Randy White of Tampa, Fla., who recently divorced and have said little publicly. Grassley's office said representatives of the Whites have indicated they will cooperate and their letter does not set a deadline for a response.

While Grassley has emphasized he is not interested in theology, Copeland and Dollar in particular have questioned whether he is targeting the ministries' shared prosperity theology. That teaching says God wants people to flourish financially and spiritually.