President Barack Obama plans a combative response if, as White House aides fear, Democrats lose Tuesday’s special Senate election in Massachusetts, close advisers say.

“This is not a moment that causes the president or anybody who works for him to express any doubt,” a senior administration official said. “It more reinforces the conviction to fight hard.”

A defeat by Martha Coakley for the seat held by the late Edward M. Kennedy would be embarrassing for the party — and potentially debilitating, since Democrats will lose their filibuster-proof, 60-vote hold on the Senate.

A potential casualty: the health care bill that was to be the crowning achievement of the president’s first year in office.

The health care backdrop has given the White House a strong incentive to strike a defiant posture, at least rhetorically, in response to what would be an undeniable embarrassment for the president and his party.

There won’t be any grand proclamation that “the era of Big Government is over” — the words President Bill Clinton uttered after Republicans won the Congress in the 1990s and he was forced to trim a once-ambitious agenda.

“The response will not be to do incremental things and try to salvage a few seats in the fall,” a presidential adviser said. “The best political route also happens to be the boldest rhetorical route, which is to go out and fight and let the chips fall where they may. We can say, ‘At least we fought for these things, and the Republicans said no.’”

Whatever words Obama chooses, however, will have trouble masking the substantive reality: A Massachusetts embarrassment would strongly increase the pressure Obama was already facing to retreat or slow down the “big bang” agenda he laid out a year ago.

Democratic operatives on Capitol Hill have made clear that enthusiasm is cooling for tackling controversial cap-and-trade legislation to curb carbon emissions as the party heads into an election year. The same is true for the always-sensitive issue of immigration reform. On the fiscal front, massive deficits were already pushing Obama toward more austerity on spending.

Perceptions among the pundit class would also be brutal. An upset by Republican Scott Brown would be covered in many quarters as a repudiation of Obama, especially after Obama’s last-ditch campaign appearance with Coakley 36 hours before the polls opened.

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