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

Sen. Max Baucus of Montana joined with Iowa Republican Sen. Charles Grassley in urging cooperation from four ministries. According to a statement from the senators Wednesday, the four ministries have either refused or failed to respond to questions Grassley first posed in early November.

Letters to the ministries from Grassley and Baucus set a March 31 deadline for the groups to turn over the requested information.

Having Baucus on board is crucial because Grassley, as the committee's ranking Republican, would need him to seek subpoenas forcing answers from uncooperative ministries.