Updated 3:52 p.m. | The outrage over the IRS’ selective review of conservative groups grew more bipartisan Monday, with numerous Senate Democrats joining GOP calls for an investigation.

Senate Finance Chairman Max Baucus, D-Mont., said his panel will probe the matter after reviewing an inspector general’s report on the targeting of applications from tea party groups for non-profit status.

“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 in a statement.

“The American people have questions for the IRS and I intend to get answers.  I want to review the Inspector General’s report first, but the IRS should be prepared for a full investigation into this matter by the Senate Finance Committee,” Baucus said. “The IRS will now be the ones put under additional scrutiny.”

Back in 2010, Baucus asked the IRS to look into allegations that groups claiming tax-exempt status were conducting political operations in violation of federal tax law.

Sen. Joe Manchin III, D-W.Va., put his views rather succinctly in a Monday statement.

“The actions of the IRS are unacceptable and un-American. Government agencies using their bureaucratic muscle to target Americans for their political beliefs cannot be tolerated,” Manchin said. “The President must immediately condemn this attack on our values, find those individuals in his Administration who are responsible and fire them.”

At the end of last week Sen. Carl Levin, D-Mich., said his Homeland Security and Governmental Affairs subcommittee would also review the matter.

“There’s no excuse for ideological discrimination in our system. The Administration should take swift action to get to the bottom of this to ensure those responsible for misconduct are held accountable and establish appropriate safeguards to prevent this from ever happening again,” Sen. Tim Kaine, D-Va., said in his own statement.

On the Republican side, Sen. Marco Rubio of Florida on Monday called President Barack Obama and Treasury Secretary Jacob J. Lew to call for the resignation of the head of the IRS.

“Furthermore, it is clear the IRS cannot operate with even a shred of the American people’s confidence under the current leadership. Therefore, I strongly urge that you and President Obama demand the IRS Commissioner’s resignation, effectively immediately,” Rubio wrote in a letter to Lew. “No government agency that has behaved in such a manner can possibly instill any faith and respect from the American public.”

Douglas Shulman, the IRS commissioner at the time of the reported inappropriate conduct by IRS officials in Cincinnati, Ohio, left his post in November, when his term expired. The IRS commissioner is appointed for a fixed term of five years. The position is subject to Senate confirmation. Acting IRS Commissioner Steven Miller is a longtime agency official.

Update 3:52 p.m.

Senate Minority Leader Mitch McConnell, R-Ky., has joined Rubio’s call for the acting IRS commissioner to step aside. McConnell told the National Review that Miller “should resign.”

“This is a lot bigger than just one person. This a whole effort by the administration, across the board, to squelch their opponents, to shut them up, and, finally, they’ve done it in a way that will allow us to call attention to it nationwide,” McConnell said.