This from a recent CNN.com article:

WASHINGTON (AP) -- Looking past the presidential nomination fight, Democratic leaders quietly fret that Sen. Hillary Rodham Clinton at the top of their 2008 ticket could hurt candidates at the bottom.

They say the former first lady may be too polarizing for much of the country. She could jeopardize the party's standing with independent voters and give Republicans who otherwise might stay home on Election Day a reason to vote, they worry.

In more than 40 interviews, Democratic candidates, consultants and party chairs from every region pointed to internal polls that give Clinton strikingly high unfavorable ratings in places with key congressional and state races.

...most Democratic leaders agreed to talk frankly about Clinton's political coattails only if they remained anonymous, fearing reprisals from the New York senator's campaign. They all expressed admiration for Clinton, and some said they would
publicly support her fierce fight for the nomination -- despite privately held fears.

The chairman of a Midwest state party called Clinton a nightmare for congressional and state legislative candidates.

I've been worried about this issue for quite some time now. I think that both Obama and Edwards (who can get away with being very progressive by being a Southerner) would give the '08 Democratic ticket a much better appeal in though races, and therefore increase the Democratic majorities in Congress by a much more substantial margin than Clinton would.