MCALLEN, Texas (Reuters) - If Sen. Hillary Clinton fears she may be on the brink of losing her bid to become the first woman U.S. president, she is determined not to show it.

Democratic presidential candidate Senator Hillary Clinton speaks at a campaign event in McAllen, Texas February 13, 2008. REUTERS/Jessica Rinaldi

The former first lady, once the red-hot favorite, has suffered a series of big losses to her rival for the Democratic Party nomination, Sen. Barack Obama, that have rattled supporters and helped prompt a shake-up in her staff.

But though tired and battling a cough picked up during weeks of endless campaigning days, Clinton is showing much of the stoicism she displayed during her sometimes troubled years in the White House as wife to President Bill Clinton.

The New York senator has plowed ahead with a relentlessly upbeat message about the state of her campaign, brushing off the predictions of pollsters who say it is in serious trouble.

“I’ve heard it before. I heard it before New Hampshire and before Super Tuesday,” Clinton told an ABC television affiliate on Tuesday.

“This is a close race. I’m very optimistic,” she said.

She was speaking several hours before Obama’s big win in three contests in the Washington D.C. area on Tuesday as he appeared to gain momentum in the close state-by-state race to represent the party in the November election.

There have been some signs that her supporters are indeed nervous and indications of some confusion in a campaign machine that had looked almost invincible last year. Some staffers have quietly acknowledged disappointment in recent results while also pressing the message that upcoming contests in Texas and Ohio will give Clinton the opportunity to rebound.

In an incident that puzzled her entourage of traveling reporters, the campaign on Tuesday called together the press for an undisclosed midday event in the Washington area, then abruptly canceled it.

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Clinton has also struggled to keep pace with the millions Obama has been raising in contributions to fund an advertising blitz and this month loaned her campaign $5 million of her own money.

CLINTON: DON’T COUNT ME OUT

When asked if she was worried that Obama may be becoming unstoppable, Clinton cited her surprise win in the January 8 New Hampshire contest as showing that polls and pundits were often wrong and it would be a mistake to count her out.

In trying to make the case that she is the stronger Democratic candidate to vie against the likely Republican candidate, Vietnam war hero Sen. John McCain, in November, Clinton often refers to herself as battle-scarred and tested.

The period in which her stoicism and determination was most on display was in the scandal the erupted over her husband’s affair with a young White House aide, Monica Lewinsky. Republicans pressed for the president’s impeachment.

The then-first lady eschewed the part of the wounded wife and instead went into full battle mode to fight the impeachment charges. She was seen as a key force in helping revive her husband’s political fortunes.

Before Tuesday’s voting was over, Clinton was in Texas, whose primary on March 4 is considered a must-win for her. The state has a large Latino population which the campaign believes may offer an important base of support.

At a rally of 12,000 people in El Paso Clinton used what was left of her hoarse voice to shout that she was excited to kick off the Texas campaign. “I’m tested, I’m ready, let’s make it happen,” she said.

But as she spoke, news had just broken that her deputy campaign manager Mike Henry, was departing.

On Sunday Clinton had announced she was replacing her campaign manager Patti Solis Doyle with another longtime aide. Clinton aides played down the move as a change in the “day-to-operations” and not an overhaul of the campaign.

In El Paso, Clinton did not publicly acknowledge Obama’s victory but the next day, she spoke about it with reporters.

But in McAllen, on the southern tip of Texas, on Wednesday Clinton -- wearing a yellow blazer that reflected the sunny demeanor she was trying to project -- said the heavy losses in the Washington area had been expected.

Clinton also offered congratulations to her opponent. “It’s been challenging, it’s been intense,” she said, while adding, “We go on. We are ready for the contests that are upcoming.”