Supporters of a controversial campaign finance reform bill are working the Senate floor and the grass roots in Massachusetts to persuade GOP Sen. Scott Brown to break ranks to help give them the 60 votes that they need to pass it.

Despite a crowded agenda, Democratic leaders want to move the DISCLOSE Act this work period in order to blunt the effect this November of the Supreme Court’s decision in the Citizens United case that cleared the way for unlimited corporate and union spending on elections.

Democrats would like to take up the bill as a stand-alone measure, but there has been discussion of attaching it to another must-pass bill if Majority Leader Harry Reid (D-Nev.) doesn’t have enough floor time for a full-on debate before August, aides said.

Reid would need 60 votes to break a GOP filibuster on the bill, and Democratic Sens. Dianne Feinstein (Calif.) and Frank Lautenberg (N.J.) have already vowed to oppose it.

Democrats hope that if Brown breaks ranks, GOP moderates such as Maine Sens. Olympia Snowe and Susan Collins will follow and provide the necessary 60 votes.

“No matter how you slice it or dice it, Scott Brown is probably the make-or-break vote,” a senior Democratic aide said.

Brown hasn’t shown any willingness to support the bill. On Tuesday, Brown spokeswoman Gail Gitcho said that hasn’t changed.

“Sen. Brown views the DISCLOSE Act as overtly political and that it changes the rules midway through the game to provide a tactical advantage just before an election cycle,” Gitcho said.

Backers of the legislation plan to make the case over the coming weeks that 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).

The message, the Democratic leadership aide said, boils down to, “Is Scott Brown going to be supporting a Supreme Court decision that makes Swift Boat ads permanent?”

The plan for pushing Brown, who has demonstrated a willingness to break with the GOP during his five-month tenure in the Senate, is based on a relatively simple dual-track messaging strategy.

In the Senate, the outreach to Brown will be led by Democratic Senatorial Campaign Committee Chairman Bob Menendez (N.J.), Judiciary Chairman Patrick Leahy (Vt.), Sen. Russ Feingold (Wis.) and Democratic Conference Vice Chairman Charles Schumer (N.Y.). Sen. Ron Wyden (Ore.), who authored provisions in the McCain-Feingold campaign finance law requiring candidates to include language explicitly endorsing their advertisements, will also be involved.

According to an internal Democratic strategy memo, these lawmakers will weave language specifically aimed at Brown into their messaging. “The path to 60 goes through him,” the memo states, outlining the arguments that Democrats will make over the next several weeks.

The memo notes that his fellow Northeastern Republican, Rep. Mike Castle (Del.), played an instrumental role in passing the bill in the House because of his “willingness to break with his party to impose common-sense limits on the potential for special-interest dollars to flood the political process. In the Senate, the question will be whether Senator Brown can also rise to the occasion or will he blindly follow his party’s leadership.”

The Democrats are targeting Brown’s image as an independent broker, which he used successfully in his upstart campaign to replace the late Sen. Edward Kennedy (D) in January.

Democrats will also tie opposition to two names that they hope will leave a sour taste in the mouths of Massachusetts voters — the Swift Boat Veterans for Truth and former Bush White House adviser Karl Rove.

Brown’s opposition to the bill would mean he is “bow[ing] to the special interests and to the Senate Republican leadership that Senator Brown claims have no hold on him,” the memo states, ending with the explicit argument that tagging him with Rove and the Swift Boat ads could make his 2012 re-election in Democratic-leaning Massachusetts difficult.

“If Brown votes against closing the Citizens United loophole, he’ll be protecting Karl Rove’s ability to run Swift Boat-style attack ads across the country this fall. That will be very difficult for a Massachusetts senator to defend,” the memo states.

At the same time, a coalition of outside organizations, including Public Citizen, the League of Women Voters, Democracy 21 and People for the American Way, has swung into action.

On Monday, some of these groups sent Brown a letter urging him to support the legislation.

“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 groups wrote. “This critical ‘accountability and transparency’ legislation deserves your support. Any effort to filibuster the Disclose Act deserves your opposition.”

League of Women Voters President Elisabeth MacNamara is in Brown’s home state this week giving interviews and conducting events urging Brown to back the bill, and People for the American Way sent a letter Tuesday to voters in the state urging them to call or write Brown to ask him to back the bill.

“We can’t expect Scott Brown to [be] the progressive champion that his predecessor Sen. Edward Kennedy was, but we can tell him we expect him to fight for the rights of his constituents over the ‘right’ of corporations to overwhelm our democratic system with unlimited and unregulated spending,” the letter states.