Sen. Scott Brown is pushing back against intense lobbying by outside organizations and Democrats trying to win his support for a controversial campaign finance bill, which the Massachusetts Republican contends would unfairly help Democrats and distract from efforts to address the economy.

With Democratic Sens. Frank Lautenberg (N.J.) and Dianne Feinstein (Calif.) opposing the bill, supporters have labeled Scott the “make or break” vote needed to move it this year. The bill, known as the DISCLOSE Act, would counteract a Supreme Court decision from January that cleared the way for unlimited corporate and union spending on elections.

The organizations Campaign Legal Center, Public Citizen, Democracy 21, League of Women Voters and Common Cause urged Brown on Monday to back the bill. “Your vote on the Disclose Act will be a vote on the most important government integrity reform measure to be considered thus far by the Senate in this Congress,” the public interest groups wrote. “This critical ‘accountability and transparency’ legislation deserves your support. Any effort to filibuster the Disclose Act deserves your opposition.”

But Brown reiterated his opposition to the measure in a letter to the coalition Wednesday, along with a detailed critique of the legislation.

For instance, Brown argued that the bill’s changes to corporate spending would end up helping Democrats in the November election. The legislation, he said, would change “the rules in the middle of the game to provide a tactical advantage to the majority party.”

Brown also took issue with special exemptions for labor unions, the National Rifle Association and others from the bill’s spending limits. He noted that labor unions in particular have vowed to spend as much as $100 million in this election cycle, “yet they would be carved out of this legislation and not face the same regulations that would apply to everyone else.”

Brown said the Senate should drop the issue for the year in order to focus on other issues. “Furthermore, this bill is being pushed when our country has almost double digit unemployment,” he wrote. “We must return our focus to job creation.”

Brown’s campaign to succeed the late Sen. Edward Kennedy (D) was based on the argument that he would act as an independent, and he has demonstrated a penchant for defying his leadership during his five months in office. He has backed a number of Democratic proposals to address the economy, and he announced this week that he would support a financial regulatory overhaul after Democrats made changes to it.

But he left little doubt Wednesday as to where he stands on the campaign finance measure.

“I understand that your five groups support this legislation, and I respect your opinion, but more than 450 other groups from across the political spectrum — ranging from such ideological opposites as the National Right to Life Committee and the ACLU — oppose this bill,” he added. “They recognize that it is based on partisan politics instead of sound policy. I could not agree more.”

Nevertheless, Democrats have said they will try to push Brown into the fold through a combination of personal lobbying and messaging efforts aimed at Massachusetts. They also plan to make the case over the coming weeks that his opposition is tantamount to supporting the kind of “Swift Boat” ads that helped sink the 2004 presidential campaign of Brown’s home-state colleague, Sen. John Kerry (D).

But Brown spokeswoman Gail Gitcho warned it will not help.

“If anybody knows anything about being ‘Swiftboated,’ it’s Scott Brown,” Gitcho said. “The Democrats ran ugly ads and mail against him filled with flat-out lies. None of the groups now lobbying for his vote came to his defense during the campaign that was fueled by left-wing special interests.”