The notion that Hillary Clinton is a "winner" who knows how to take on the right wing is ridiculous, but she keeps touting it:
“I’ve noticed in the last few days that a lot of the other campaigns have been using my name a lot,” Mrs. Clinton said. “I’m here because I think we need to change America; it’s not to get into fights with Democrats.”
“For 15 years, I have stood up against the right-wing machine and I’ve come out stronger, so if you want a winner who knows how to take them on, I’m your girl.”
Actually, it was Bill Clinton who knew how to take them on. Hillary was not a "winner" but a presidential spouse/adviser whose public image and poor political instincts held her husband back. She only became a sympathetic figure who helped Bill after the Monica Lewinsky affair became public. And her only significant political victory was defeating Rick Lazio in 2000, a race in which she performed about as well as Chuck Schumer did in 1998. In short, her entire claim to being a "winner" is to do about as well as a generic Democrat in a Democratic-leaning state. It's not so impressive. (Or if it is, why isn't Chuck Schumer running for president on his record of defeating Republicans?)
In fact, one thing that's striking about the current Democratic field is how few tough political races they've won. Bill Clinton won a number of competitive races in Arkansas before entering presidential politics. However, the top three contenders for the Democratic nomination have a combined total of two wins in competitive general elections above the state legislative level (Hillary's Senate victory in 2000 and Edwards's in 1998) and only four wins total (add on Obama in 2004 and Hillary's re-election in 2006). Only one of those -- Edwards's win in 1998 -- came in unfavorable territory.
By contrast, the top three Republicans at this point have three wins in competitive generals (Giuliani's first mayoral election in 1993, Fred Thompson's first Senate election in 1994, and Mitt Romney's 2002 election as governor) and five wins total (add Thompson and Giuliani's re-elections in 1996 and 1997, respectively), with both Giuliani and Romney winning on hostile turf.
If you believe UCLA political scientist John Zaller that politicians are like prizefighters, Democrats might be in trouble (PDF):
World heavyweight champions, like [members of Congress], compete in a sequence of contests in which the winner continues for as long as possible to fight new challengers, most of whom have never previously been a champion. Incumbent champions in both professions are thus a much more selected group than their opponents. This structural similarity creates a likelihood that the champions, whether boxers or MCs, will be, in some general sense, "better" competitors than most challengers and may seem to enjoy a special incumbency advantage for this reason alone.
Of course, the political environment may be so favorable to Democrats that any top candidate could win, but let's just say that the contenders for the championship belt are not exactly proven "winners".
Correction 8/9 9:11 AM: This post previously said the top three Democratic presidential contenders had won three times above the state legislative level. In fact, they've won four times. The error has been corrected above.