Add a prominent Senate Democrat to the list of congressional leaders seeking hearings into the Internal Revenue Service's treatment of Tea Party groups seeking tax-exempt status.

Senate Finance Committee Chairman Max Baucus, D-Mont., said his committee's investigation would include "who knew what" about the IRS's extraordinary scrutiny of conservative groups. "The IRS will now be the ones put under additional scrutiny," Baucus said in a written statement to reporters Monday.

"These actions by the IRS are an outrageous abuse of power and a breach of the public's trust. Targeting groups based on their political views is not only inappropriate but it is intolerable," Baucus said.