Minority Leader Mitch McConnell (Ky.) said the Obama administration has shown “an alarming willingness itself to use the powers of government to silence” political speech of groups with which it disagrees. He cited a Democratic proposal to require corporations and unions to disclose their spending on political advertising. (Chip Somodevilla/Getty Images)

The Senate’s top Republican accused President Obama and congressional Democrats of trying to restrict opponents’ political speech.

In a speech Friday at the American Enterprise Institute, Minority Leader Mitch McConnell (Ky.) said the Obama administration has shown “an alarming willingness itself to use the powers of government to silence” political speech of groups with which it disagrees.

“It is critically important for all conservatives — and indeed all Americans — to stand up and unite in defense of the freedom to organize around the causes we believe in, and against any effort that would constrain our ability to do so,” McConnell said in the speech at AEI, a Washington group that says it supports free enterprise.

McConnell, long an opponent of restrictions on political contributions, cited a Democratic proposal to require corporations and unions to disclose their spending on political advertising.

He said it would require “government- ­compelled disclosure of contributions to all grass-roots groups, which is far more dangerous than its proponents are willing to admit.”

“This is nothing less than an effort by the government itself to expose its critics to harassment and intimidation, either by government authorities or through third-party allies,” McConnell said.

Democrats proposed the disclosure measure in response to a 2010 Supreme Court decision overturning a decades-old ban on companies using general funds to run ads supporting or opposing federal candidates.

The ruling led to the rise of political organizations known as super PACs, which can raise unlimited money from any source. A number of super PACs have formed to influence the 2012 presidential and congressional races.

The House passed the measure in 2010, when Democrats controlled the chamber. The legislation did not advance in the Senate, where all the Republicans opposed it.

The bill’s chief sponsor, Sen. Charles E. Schumer (D-N.Y.), held a March hearing on a scaled-back version and has said he may seek another vote on it before the November elections.

McConnell singled out the Internal Revenue Service for criticism.

“Earlier this year, dozens of tea party-affiliated groups across the country learned what it was like to draw the attention of the speech police when they received a lengthy questionnaire from the IRS demanding attendance lists, meeting transcripts and donor information,” he said.

The IRS has denied that it selects groups for scrutiny based on their political views.

IRS Commissioner Douglas H. Shulman said March 21 that the tea party groups had applied for nonprofit status and could have operated as nonprofits without seeking IRS approval first.

“There’s many safeguards built in so this has nothing to do with election cycles and politics,” he told a House Appropriations subcommittee. “This notion that we’re targeting anyone is off.”