ATLANTA - Hardly the start he'd hoped for.

Newt Gingrich's first week as a presidential candidate has been riddled with missteps that angered fellow Republicans and exposed campaign vulnerabilities.

The former U.S. House speaker disparaged House Republicans' proposal on cutting Medicare costs as "right-wing social engineering" and was all but forced to apologize after the outcry.

He tied himself in knots as he defended a key aspect of the Democrats' health-care law - while saying he opposes the law.

And he refused to explain how he came to once owe $500,000 to the upscale Tiffany's jewelry store, though he rails against President Obama for what he calls excessive federal spending.

"He has severely damaged his campaign and his credibility," said Debbie Dooley of Duluth, Ga., a national coordinator for the Tea Party Patriots who contended that Gingrich made things worse when he tried to explain why he favored requiring Americans to have health insurance.

"If he continues with that position, for the most conservative tea party Republicans . . . it's over," she said.

Advisers say Gingrich has proven he can survive such troubles.

"The base and the insiders are paying attention, and certainly Speaker Gingrich got their attention - and possibly in a bad light last week," said Katon Dawson, a supporter and former chairman of the South Carolina GOP.

But Dawson argues that voters haven't yet tuned in, so the damage is minimal.

"People have counted Newt Gingrich out on numerous occasions before at their own peril," he said.

Gingrich hopes his tenure in the 1990s as the top House Republican will lend him credibility with establishment Republicans searching for a candidate to back in a muddled field, and that the time he spent after he left office building a grassroots network of conservative activists will gain him tea party support.

His fumbles underscored a big hurdle he must clear to win the nomination: himself. The missteps renewed doubts about whether he has the discipline to be a candidate, much less president.

It all started Sunday on NBC's Meet the Press in an interview that was part of his campaign rollout.

Gingrich disparaged Republican Rep. Paul Ryan's Medicare proposal, which is popular with the party's right flank, as "too big a jump."

He also gave an inconsistent response when confronted with his past support for a mandate that Americans be required to buy health insurance, a central part of Obama's health-care overhaul that conservatives despise.

Outcry was so fierce that Gingrich's camp convened two conference calls to soothe tea party activists and he called Ryan to apologize.

As if those hiccups weren't enough:

An Iowa voter confronted him and urged him to end his White House run "before you make a bigger fool of yourself." The moment, captured on video, has made its way around the Internet.

A Minnesota voter showered the candidate and his wife, Callista, with glitter in a bizarre piece of political theater to protest Gingrich's opposition to gay marriage.

For some in his party, his remarks on the Ryan plan crossed a line.

"He seemed to be saying what he believed," said Julianne Thompson, head of Georgia Tea Party Patriots who thinks Gingrich's campaign may be on life support.