Republican congressional leaders are calling on the newly appointed IRS commissioner to scrap a new rule they claim would target politically active groups seeking tax-exempt status -- including the very same conservative groups the agency is accused of harassing.

"This proposed rule is an affront to free speech itself," the lawmakers wrote in a letter to IRS Commissioner John Koskinen.

The letter -- signed by House Speaker John Boehner, Senate Republican Leader Mitch McConnell and others -- marks the most coordinated effort yet by the GOP to fight the rule change, first proposed in November.

The letter comes as more conservative groups testified Thursday about IRS targeting, before a House oversight subcommittee.

"They were harassed at the hands of their very government," Rep. Jim Jordan, R-Ohio, said.

More On This...

The central complaint over the proposed rule is that it would effectively codify the kind of scrutiny that Tea Party and other groups were subjected to before the 2010 and 2012 elections. Those groups had been applying for what's known as 501(c)(4) tax-exempt status, as "social welfare" organizations. The rule change would limit the kinds of political activity that "social welfare" groups -- of any ideological leaning -- could engage in.

The proposal was described by the administration as a way to clarify the rules, and create "clear-cut definitions of political activity."

But Republicans now say the rule change would allow political targeting to continue unabated.

"It is our view that finalizing this proposed rule would make intimidation and harassment of the administration's political opponents the official policy of the IRS and would allow the Obama Administration to use your agency as a partisan tool," Republicans wrote in their letter to Koskinen.

A day earlier, House Oversight and Government Reform Committee Chairman Darrell Issa, R-Calif., and Rep. Jordan also urged Koskinen to withdraw the regulation.

The new IRS commissioner was asked about the regulation at a hearing Wednesday before the House Ways and Means Committee. He said he would look at it "independently," noting that it already had a record 21,000 comments.

Committee Chairman Dave Camp, D-Mich., also claimed that new emails show IRS officials, including Lois Lerner, discussed these kinds of regulations privately as far back as 2011 and 2012 -- even though they were initially cast as a response to the targeting scandal. Camp said the changes were floated as "another line of attack against these groups."

Koskinen, though, said he was not aware of "the background of how the regulations were drafted."

Republicans have raised myriad concerns in recent weeks about the status of the IRS controversy. Lawmakers tried to call Justice Department official Barbara Bosserman to testify on Thursday before the House oversight subcommittee, but Bosserman did not show up. The official is leading the probe into the IRS, but is also a political contributor to President Obama, and Republicans have questioned her objectivity.

Obama, meanwhile, has downplayed the scope of the scandal.  He told Fox News' Bill O'Reilly on Sunday that there was "not even a smidgen of corruption" at play.

Appearing to defend the proposed rule change, he said IRS officials subjecting conservative groups to additional scrutiny over the past several years were simply confused about how to implement the law.