Boston ? His agenda at risk, President Barack Obama fought Sunday to save a struggling Democratic U.S. Senate candidate and the critical 60th vote needed for his health care plan. The White House and congressional Democrats scrambled to find a way to pass the bill quickly if Martha Coakley loses a special election Tuesday.

“Understand what’s at stake here, Massachusetts. It’s whether we’re going forward or going backwards,” Obama said during a rally for Coakley as he tried to energize his dispirited base in this Democratic stronghold. “If you were fired up in the last election, I need you more fired up in this election.”

The president made a direct appeal to independents who are trending away from the Democrat and sought to court voters angry over Wall Street abuses. He assailed GOP candidate Scott Brown, who downplayed his party affiliation during the campaign, as just another typical Republican who sides with special interests.

The unexpectedly tight race for the seat held so long by Edward M. Kennedy, in a state where registered Democrats outnumber Republicans by 3-to-1, reflects a nasty antiestablishment environment that threatens Obama’s support in Congress now and heading into this fall’s elections.

Brown, a little-known state senator, has tapped into voter anger and anxiety over budget-busting spending, expanded government and high unemployment under Obama to pull even with Coakley, the state’s attorney general.

“It’s us against the machine,” Brown said in Worcester, alongside former Boston College football star Doug Flutie and former Red Sox pitcher Curt Schilling. “The establishment is afraid of losing their Senate seat. You can all remind them that this is not their seat, it is yours.”

If elected, Brown says he would vote against Obama’s health care bill, robbing Democrats of the 60-vote majority needed to prevent Republicans from blocking it and other parts of Obama’s agenda.

In Washington, White House aides and Democratic lawmakers frantically hashed out plans to save the health care bill in case of a Brown upset. The likeliest scenario emerging would require House Democrats to accept a bill the Senate passed last month, despite their objections to several parts. Obama could sign it into law without another Senate vote needed. House leaders would urge the Senate to make changes later under a complex plan the would require only a simple majority.