Hillary Clinton is the most famous woman in Americans politics, backed by the Democratic Party's most powerful figures and richest donors. Why then is she dropping so dramatically in the polls as she heads into the party's first presidential debate?

“At the beginning of the year she was the most admired person in public life," her husband, former president Bill Clinton, said recently. "What happened? The presidential campaign happened."

It happened hard.

As her husband proudly points out, an annual Gallup poll named Hillary Clinton as the woman Americans admire most, just as she has been almost every year for more than two decades.

That enduring admiration is either evaporating or irrelevant. A USA Today/Suffolk University poll found that 59% of likely Democratic voters were behind her in July. This month, that number dropped to 41%.

The Democrat drawing the biggest crowds isn't Clinton, but rather Vermont Senator Bernie Sanders, a socialist who spent decades on the fringes of American public life. The Democrat generating the most suspense is Vice President Joe Biden, who's trying to decide whether or not he'll also run. Biden's aides have let it be known that he won't be at the debate. But it's an unprecedented opportunity for the candidates who will.

The audience across all media - on CNN Tuesday night or early Wednesday, depending on where you're watching - is almost certain to be in the millions. It will be the biggest one ever to hear Sanders, former Maryland governor Martin O'Malley, former US senator Jim Webb or former Rhode Island governor Lincoln Chaffee.

It also represents a big opportunity for Clinton, to generate some excitement and change the conversation about her campaign, a conversation dominated for several months by a complicated controversy about her email.  Unlike virtually all US government employees today, Clinton used personal email addresses channelled through a privately-owned computer server during her four years as secretary of state during President Barack Obama's first term.

After Republican lawmakers pushed for access to her official correspondence, Clinton's lawyers gave the government 55 000 pages of emails, which have been gradually reviewed and released to the public. Whether or not Clinton knew it at the time, a small number of them contained information now considered secret. Entrusting them to a private server without the benefit of sophisticated encryption may have been a security breach.

Clinton eventually publicly apologized, but she blames Republicans for turning the emails into an election issue and at least one powerful Republican agreed.

"Everybody thought Hillary Clinton was unbeatable," said Kevin McCarthy, Republican majority leader in the House of Representatives. "What are her numbers today? Her numbers are dropping. Why? Because she's un-trustable. But no one would have known any of that had happened had we not fought and made that happen."

McCarthy has since backed away from the remark and said that the Republicans' interest in Clinton's emails was part of an ongoing investigation with no partisan purpose.

But the hunt for emails has hurt her. The USA Today poll found that 70% of voters think it diminishes her chance of winning the White House.

As a former first lady, senator and secretary of state, Clinton has more experience than any other candidate in the race. She is a familiar face to the American people, with many admirers, but dogged by doubts and detractors.

She has to start over somehow and find some new kind of spark. The debate will give her a chance.

Written by Jonathan Mann, Anchor, CNN International