U.S. Rep. Michele Bachmann says the IRS targeting of Tea Party and conservative groups that sought tax-exempt status during the 2012 election cycle is "far worse" than the Watergate scandal of the Nixon presidency.

During a press conference Thursday on Capitol Hill, Bachmann and other tea party leaders demanded a thorough investigation of the IRS and the Obama administration's possible role in the controversy, which led the president to dismiss acting director Steven Miller; his resignation is effective next month.

"We haven't even scratched the surface, we haven't even begun," said Bachmann, chairwoman of the House Tea Party Caucus and a former IRS attorney.

Second later she said: "This is far worse than Watergate ... These are direct actions taken against American citizens who sought to exercise their free speech rights under the First Amendment."

Last week, the IRS admitted singling out groups with the words "tea party" or "patriot" in their 501(c)(4) applications required for groups that want to operate as social welfare organizations.

Leaders with the Rochester Tea Party Patriots in southern Minnesota said this week it took the IRS more than two years to approve their tax-exempt status.

The IRS has said that roughly 75 groups were targeted during an 18-month period that ended in summer 2012. But officials with another Minnesota group, Minnesota Majority, said they were targeted for extra scrutiny this spring.

"Progressive liberal groups in line with the administration were fast-tracked ... where the Tea Party groups were told to sit in the corner and on the curb and they were denied or delayed," Bachmann said.

Not all conservative groups were targeted. Citizens for Responsibility and Ethics in Washington said that the American Action Network, a group led by former Minnesota U.S. Sen. Norm Coleman, had their application approved in less than six weeks. The group raised more than $22 million during the last election cycle.

The IRS did not deny tax-exempt status to any of the tea party organizations seeking tax-exempt status, but more than two dozen agencies withdrew their applications after receiving extra scrutiny, officials said.

Flanked by leaders of more than a dozen tea party groups from across the country, Bachmann said that many organizations couldn't take contributions last election cycle because their applications were intentionally stalled.

"The IRS essentially was withholding the ability" of the Tea Party movement to have a voice, she said.

"We don't know how much impact that had on the outcome of the last election, but we aim to find out."

On Friday, Republican U.S. Rep. Erik Paulsen, a member of the House Ways and Means Committee, will participate in the first hearing on the IRS matter when the panel questions Miller and the Treasury Inspector General J. Russell George.