Sen. Charles Grassley, the top Republican on the Senate Finance Committee, is preparing another round of letters to Christian television ministries, prodding them to answer questions about their spending and the way they are governed, a spokeswoman said Thursday.

The Iowa Republican started his investigation of the ministries in early November but has met resistance from some of the six ministries he has questioned.

The senator's latest attempt to get answers from the holdouts comes as criticism of his probe is mounting within the evangelical establishment. The flagship magazine of the movement, Christianity Today, editorialized this month that the Grassley probe amounts to "oversight overstep" that risks delving improperly into theology.

Others — including the head of the Evangelical Council for Financial Accountability and the editor of Charisma, another evangelical magazine — support Grassley but say they've been pressured to change their positions.

The targeted ministries have gotten advice from Jay Sekulow, a Washington lawyer well-connected in evangelical and Republican circles, according to two sources with knowledge of the talks who requested anonymity because they are not authorized to speak publicly about them.

Sekulow heads the American Center for Law and Justice, founded by televangelist Pat Robertson.

A statement issued by the center said: "While the ACLJ is concerned about the constitutional issues raised by the inquiry, it will not be making any appearances on behalf of any of the ministries before the Senate Finance Committee and is not going to comment further on what counsel it has given or not given to any of the ministries involved."

In early November, Grassley sent letters to six Christian ministries with dozens of questions about salaries, mansions, private jets, expensive furniture and board makeup.

While Grassley has emphasized that his interest lies not in doctrine but in whether tax laws are being followed, several ministries have complained that religious freedoms are being endangered. They say the IRS, not a Senate committee, is the proper forum for investigating the complaints that Grassley says prompted his probe.

The ministries — all of which preach a "prosperity gospel" that says God wants his followers to flourish financially — also have insisted that they follow IRS rules on how they spend their donors' dollars.

Under IRS guidelines, ministers may receive "reasonable" compensation but cannot enrich themselves through their nonprofits.

Two of the ministers, Creflo Dollar and Bishop Eddie Long, both based in suburban Atlanta, have refused to cooperate with Grassley's investigation.

Missouri-based Joyce Meyer Ministries offered full cooperation and has made its documents available.

A Texas ministry led by Kenneth Copeland also has turned over information, although the extent of the group's cooperation isn't clear.

The two remaining groups, one led by Texas-based faith healer Benny Hinn and the other by Randy and Paula White of Tampa, Fla., have been in touch with Grassley's office but have not indicated whether they are cooperating or opposing.

(Today, Hinn continues his Miracle Crusade in Honolulu's Blaisdell Arena.)

Jill Gerber, Grassley's finance committee spokeswoman, said follow-up letters will go out in the next few weeks to ministries that have raised concerns or not responded.

Gerber said the letters will describe Congress' authority and duty to investigate and evaluate laws over which it has legislative authority — in this case, tax-exemption policy.

"It's been extremely rare for tax-exempt groups to decline to cooperate with his requests for information," she said in a statement to the Associated Press. "Tax-exempt groups usually work to answer questions from a leader of the Senate committee that sets tax policy. Senator Grassley hopes that these groups will be similarly cooperative with this inquiry."

Democratic Sen. Max Baucus of Montana, the finance committee chairman, has not commented publicly on Grassley's investigation. But Carol Guthrie, communications director for the committee's Democratic staff, said this week Baucus is aware of the investigation, and suggested that the ministries should cooperate.

"Customarily, people come to recognize the wisdom of responding to requests by senior members of the finance committee, and particularly by Senator Grassley, before a subpoena becomes necessary," she said.

Grassley has said he doesn't believe he'll need subpoenas. But if he seeks them, he would need Baucus' backing. Grassley has previously pointed out that when he was committee chairman, he worked well with Baucus.

Gaining support of the Democratic chairman would not only give the investigation added heft, it would deflect criticism like that of Dollar, the most outspoken critic among the ministers under scrutiny.

Dollar said last week on the Fox Business Network that it is Grassley, and not the committee, investigating him and the other preachers.