(CNSNews.com) - Grassroots conservatives want Sen. John McCain to know how strongly they oppose his continuing efforts to "muzzle political speech."

A coalition of conservative groups warns that McCain, in various public statements, has signaled his intention to introduce legislation that would ban so-called "527" political organizations.

In a letter to McCain, conservatives expressed their emphatic opposition:

"The irony, of course, lies in the fact that the '527' organizations you now seek to silence are a direct consequence of McCain/Feingold's abridgments of free political speech," the letter said.

The 2002 McCain-Feingold campaign finance law restricts political speech in the run-up to elections. Corporations, labor unions and nonprofits are barred from mentioning in TV and radio ads the names of candidates for federal office within 30 days of a primary election and within 60 days of a general election.

That gave rise to the 527 groups, which are named after the applicable section of U.S. tax code. Some examples include MoveOn.org, Swift Boat Veterans for Truth, and hundreds of other groups on both the political left and right.

The 527s are created to raise money from any source, and those funds are then spent with the intention of influencing the outcome of elections. Unlike political action committees, the 527s are not regulated by the Federal Election Commission.

"We hold the 2002 McCain-Feingold legislation to be one of the greatest legislative assaults on the First Amendment ever passed by Congress," said the letter signed by conservative groups. "Just as we opposed that legislation, so too do we oppose any further restrictions on political speech."

The 18 groups that signed the letter also noted McCain's unwillingness to limit his own speech as he campaigns for the 2008 presidential nomination:

"We find it troubling that you would introduce and advocate new speech-stifling legislation at a time when it appears that your presidential campaign is willing to forego similar limitations in favor of your political aspirations," the letter reads.

"In John McCain's world, what's good for the goose is apparently not good for the gander," said Jeffrey Mazzella, president of the Center for Individual Freedom (CFIF), one of the groups that helped write the letter.

"With McCain's stated intentions to further muzzle the voices of the American people, it is clear that the only person he wishes to speak is himself," Mazzella continued. "So much for 'We the People.'"

In addition to the Center for Individual Freedom, groups represented on the letter include: American Conservative Union, James Madison Center for Free Speech, Citizens United, The National Center for Public Policy Research, Institute for Liberty, Citizen Outreach Project, Coalition for a Fair Judiciary, RightMarch.com, JunkScience.com, Americans for Prosperity - Texas, GrassTopsUSA, Taxpayers League of Minnesota, Grassroot Institute of Hawaii, Frontiers for Freedom, Liberty Coalition, GrassrootsFreedom.com and Conservative HQ.

McCain, distrusted by many conservatives since his last run for president, is making an active attempt to woo them this time around.

CFIF was a plaintiff in the U.S. Supreme Court case McConnell v. FEC, which challenged the constitutionality of the "McCain-Feingold" Bipartisan Campaign Reform Act of 2002, after President Bush failed to veto it.

CFIF has also launched a blog focused on Senator McCain's refusal to "talk straight" with the American people about his intentions to campaign for president within the public financing system.