President Obama misspoke and did not intend to mislead the public when he said AARP had endorsed his health care legislation pending in Congress, the White House said Wednesday. 

White House Press Secretary Robert Gibbs told reporters that Obama did not mean to misinform Americans when he said Tuesday at a town hall in Portsmouth, N.H., that the senior lobby had backed his legislation. 

"AARP has said they are certainly supportive and have been for years on comprehensive health reform," Gibbs said.  "I don't think the president meant to imply anything untoward. I think he discussed the notion that AARP is supportive of legislation -- or, I'm sorry, an agreement that would -- that would fund filling the doughnut hole for seniors as part of Medicare Part D, as well as additional savings for comprehensive health care reform."

Obama said at the town hall that "we have the AARP onboard because they know this is a good deal for our seniors." He added, "AARP would not be endorsing a bill if it was undermining Medicare."

AARP's chief operating officer Tom Nelson quickly fired back, saying, "indications that we have endorsed any of the major health care reform bills currently under consideration in Congress are inaccurate."

The group is sensitive to the issue because polls show that Medicare beneficiaries are worried their health care program will be cut to subsidize coverage for the uninsured.

Like Obama, AARP wants action this year to cover the uninsured and restrain health care costs, but the organization has refrained from endorsing legislation. Nelson said AARP would not endorse a bill that reduces Medicare benefits.

A spokesman said the Medicare cuts that have been proposed so far would not affect benefits.

Obama assailed "wild misrepresentations" of his health care plan during the town hall, taking on the role of fact-checker-in-chief for his top domestic priority. It's a strategy he will employ at two more town halls this week in Montana and Colorado, and on the White House Web site.

To that end, the Obama-aligned Democratic National Committee is running health care overhaul ads nationally on cable channels and in spots the president will visit, joining a chorus of ads that has become a cacophony over a problem that has vexed Washington for decades.

On the other side, the U.S. Chamber of Commerce was joining the fray Wednesday, beginning to air 30-second spots in about 20 states criticizing the Democratic proposal to offer optional government health coverage, according to R. Bruce Josten, executive vice president of the nation's largest business group.

The multimillion-dollar buy would be one of the largest so far critical of Obama's effort, in a year in which opponents have been heavily outspent by supporters of the president's plan. The spot, showing a balloon being inflated until it bursts, says: "Big tax increases, huge deficits, expanded government control of health care. Call Congress."

In Portsmouth, Obama faced a polite crowd of 1,800 packed into a high school auditorium and a nationwide audience watching on cable television. He urged them not to listen to those who seek to "scare and mislead" on his plans to overhaul the nation's health care system.

"Where we do disagree, let's disagree over things that are real, not these wild misrepresentations that bear no resemblance to anything that's actually been proposed," he said. "Because the way politics works sometimes is that people who want to keep things the way they are will try to scare the heck out of folks, and they'll create boogeymen out there."

The boogeymen have prompted the White House to strike back. The president ticked off the highest-profile, most emotional issues that critics have used to greatest advantage to interrupt town hall meetings held by lawmakers home for the August congressional recess.

For instance, Obama said the Democratic health care legislation would not create "death panels" to deny care to frail seniors -- or "basically pull the plug on grandma because we decided that it's too expensive to let her live anymore," as the president put it. The provision he said had led to such talk would only authorize Medicare to pay doctors for counseling patients about end-of-life care if they want it, he contended.

He also disputed accusations that he seeks a federally run system, or one in which the government makes decisions about care.

Obama's new message, sharpened amid sliding public support for him and his plan, targeted a vital and, polls show, particularly skeptical audience: the tens of millions of people who already have health insurance and aren't yet convinced of a need to spend billions of dollars to change it or cover the nearly 50 million people who lack coverage.

That message is finding reinforcements online. The White House launched a Web site to counter critics and asked supporters to share with them e-mails they say misrepresent Obama's positions. It's a tactic similar to the one the tech-savvy Obama campaign used to win the White House.

The Associated Press contributed to this report.