Senate Minority Leader Mitch McConnell said on CNBC's "The Kudlow Report" the IRS' "huge scandal" is just one of many instances where the Obama administration has attacked conservatives and the First Amendment.
"What you've got is administration that, based on the Attorney General's report, engaged in discriminating against conservative groups who had criticized the administration," he said. "This is a huge, huge story."
His remarks came in response to Attorney General Eric Holder's order to the FBI Tuesday to open a criminal probe in the Internal Revenue Service's targeting of conservative political groups.
(Read More: FBI to probe U.S. tax agency's actions on conservatives groups)
McConnell said the limitations on free speech extended beyond the IRS, including the Department of Health and Human Services, the Securities and Exchange Commission and the Federal Communications Commission.
"The Department of Health and Human Services, during the Obamacare debate, issued a directive to the health insurance companies telling them they could not say to their policy holders what their objections were to Obamacare," he said.
McConnell demanded more answers on the role of the White House in the IRS scandal, saying the Obama administration cannot be trusted to be forthcoming on its details.
"The truth always comes out and it will come out," he said. "We know enough already to know this was outrageous behavior."
McConnell said he sent a letter to former IRS Commissioner Douglas Shulman last March questioning selective enforcement on tax exempt organizations after receiving claims of unusual harassment by the IRS from constituents.
(Read More: Watchdog cites poor management in U.S. IRS 'Tea Party' targeting)
The Republican leader said the story would have been kept a secret had his party not demanded the truth, and he recognized his Democrat colleagues who have joined in the criticism of the IRS.
"An investigation into the IRS abuses by Congress will have a lot of credibility," he said "We have the tools to get to the bottom to what happened."
The House of Representatives Ways and Means Committee is scheduled to hold a hearing on the scandal on Friday.
—By CNBC Associate Producer Elizabeth Schulze. Follow her on Twitter: @ESchulze9