Clinton aides hit Obama ad

But the remark still reflected the emotional dispute over how best to achieve universal healthcare, a key concern of many Democratic primary voters.

The Obama mailer, which the Clinton campaign traced to mailboxes in North Dakota and Alaska, shows a couple sitting at a table, appearing to puzzle over a stack of bills. "Hillary's plan forces everyone to buy insurance, even if you can't afford it," the headline reads.

On a conference call with reporters, Clinton advisers contended that the image and the message closely resembled the $100 million "Harry and Louise" TV ad campaign waged by the insurance industry in 1994 to kill Clinton's effort to reshape the healthcare system. In those ads, a middle-class couple sat at a table worrying about the plan's complexities and wondering whether they might lose the right to choose their own doctors.

Neera Tanden, Clinton campaign policy director, said the mailer falsely suggests that the New York senator's plan wouldn't bring down costs. She said Clinton would offer tax breaks to help pay for insurance and would seek other cost controls before enforcing a mandate for coverage.

Obama, Tanden said, "betrays the cause of universal healthcare. For a potential Democratic nominee to be attacking universal care is quite stunning."

Nichols went further. "I am personally outraged at the picture used in this mailing," said Nichols, a supporter of the so-called universal mandate. "It is as outrageous as having Nazis march through Skokie, Illinois."

In the late 1970s, the American Nazi party won a court battle over the right to march through the predominantly Jewish Chicago suburb of Skokie, home to many Holocaust survivors. Despite their victory, the white supremacists decided to hold their demonstration in a Chicago park instead.

While both Clinton and Obama have outlined detailed plans to make healthcare more affordable and accessible, Clinton would require everyone to carry insurance while Obama would make enrollment voluntary. The two have clashed repeatedly over the issue, with Obama saying people cannot be required to buy insurance if they can't afford it, and Clinton saying universal enrollment is the only way to bring down insurance costs.

Bill Burton, an Obama spokesman, responded by pointing to comments by Senator Edward M. Kennedy of Massachusetts, a longtime champion of universal healthcare who endorsed Obama earlier this week.

"It's the passion of my life, universal, comprehensive healthcare, and I wouldn't support Barack Obama unless I was absolutely convinced that he was for universal, comprehensive healthcare as well," Kennedy said in a television interview.