Some analysts doubt that the number of defectors will be that large, once the Democratic campaign ends and passions cool. They add that it may not be accurate to attribute potential defections to the sometimes nasty tone of the race and that other factors, including resistance to voting for a woman or an African-American, may be more important factors.
At the same time, Clinton's image has suffered as a result of the long campaign, according to recent public opinion surveys.
An NBC News--Wall Street Journal poll late last month found that, overall, she is viewed more negatively than favorably by voters and that feelings toward her were more negative than at any time since she launched her candidacy.
Paul Johnston, a card-carrying member of the most heavily targeted group of Pennsylvania Democrats - white, working-class men - is among those who've been turned off.
"I don't trust her," said Johnston, 56, of Greensburg, a member of the International Union of Operating Engineers who expects to vote for Obama. Clinton's latest attack ad is "mudslinging. I don't like that kind of campaigning."
Clinton's chances of running up her popular vote margin in this week's primary, which her husband has identified as her campaign's top goal, depends at least in part on her ability to plant doubts about Obama among wavering voters.
To that end, Clinton's attack ad, which tries to undermine Obama's image as a different kind of politician, is "very effective," said Kathleen Hall Jamieson, a communications professor who heads the University of Pennsylvania's Annenberg Public Policy Center.
"It speaks to the core identity of the opposition candidate, which is what you want an ad to do," she said of Obama's attempt to present himself as one who won't be a tool of Washington lobbyists and special interests.
Winning by a landslide in Pennsylvania, a state where she has family ties and enjoys a number of demographic and practical advantages - including support from the state's governor and the mayors of its two largest cities - would bolster Clinton's claim to be the stronger candidate against McCain.
A longtime analyst of Pennsylvania elections, G. Terry Madonna, says he has been impressed with Clinton's tenacity and how she has turned the contest into a fight about values, rather than issues she highlighted in neighboring Ohio, such as the NAFTA trade deal.
