James Bopp Jr., a prominent free-speech advocate, lumped the former Tennessee senator with Sen. John McCain as he condemned the "McCain-Feingold-Thompson" law aimed at restricting the influence of special interest groups on the election process.
"There was no more adamant supporter of campaign finance reform and the regulation of political speech and issue ads that Senator Thompson," Bopp wrote in a column for Townhall.com, a conservative Web meeting place. "His support was not reluctant; it was enthusiastic and repeated. He actively and enthusiastically supported regulation of and, ultimately, total prohibition of corporate and labor union issue ads, which he never viewed as a 'mistake."'
While Romney aides have questioned Thompson vigor on the campaign trail, Bopp wrote: "Senator Thompson devoted much energy in the Senate to gutting the First Amendment."
Thompson released a statement noting Romney's own efforts to curtail the influence of money in politics.
"Until he started running for President, Mitt Romney had a long history of supporting campaign finance reform and restrictions even more stringent than McCain-Feingold legislation," said the statement.
During a 1994 U.S. Senate campaign, Romney proposed eliminating political action committees, favored a spending cap for his race against Sen. Edward M. Kennedy, D-Mass., and supported publicly financing elections in part by imposing a 10 percent tax on those who self-finance their races.
Eight years later, during Romney's 2002 race for governor of Massachusetts, the former venture capitalist contributed $6.3 million to his campaign, or 64 percent of the total raised. In the first half of this year, he contributed $9 million to his presidential campaign and has refused to say if he has given more during the quarter that ends Sept. 30.
Romney's decision to focus on Thompson, who only entered the race this month, highlights their battle for the hearts and minds of conservative voters.
Earlier this year, Romney focused on McCain's authorship of the 2002 fundraising law.
"I favor transparency," Romney said in April while campaigning in New Hampshire. "Let people make contributions and report it on the Web site, so you know who's contributed to whom, but McCain-Feingold has not worked. It's hurt my party, it hurts First Amendment rights. I think it was a bad bill."