WASHINGTON — As jubilant Democrats prepared for President Obama to sign their landmark health care legislation in a big ceremony at the White House, Republicans opened a campaign on Monday to repeal the legislation and to use it as a weapon in this year’s hotly contested midterm elections.

“We will not allow this to stand,” Representative Michele Bachmann, Republican of Minnesota, promised Monday afternoon as the House reconvened, a day after the bitterly partisan vote.

Democratic leaders hailed the passage of the bill as a towering achievement.

“Last night, we made history,” Speaker Nancy Pelosi said as she prepared to sign the legislation and send it to the White House. “We honored the vows of our founders who in the Declaration of Independence talked about life, liberty and the pursuit of happiness. We believe the legislation that we have gives all people in our country the liberty to have healthier lives.”

Democrats said they would focus on explaining the measure to their constituents and on highlighting some immediate benefits, and they called on Republicans to ease off on their attacks now that the legislation had passed.