WASHINGTON — A senior Democratic lawmaker says Senator Clinton’s proposed health care mandate would be a “nonstarter” in the Senate, ratchetting up the rhetoric on an issue that once united Democrats.

The criticism from Senator Kerry, the Democratic presidential nominee in 2004, of the core element of Mrs. Clinton’s domestic platform signals a sharper intraparty divide over health care policy heading into the fall.

“Let me just tell you that Hillary Clinton’s plan in the United States Senate is a nonstarter, because it starts with a mandate that is unachievable in the Senate in what we need to do,” Mr. Kerry, a top supporter of Mrs. Clinton’s rival for the Democratic presidential nomination, Senator Obama, said yesterday on ABC’s “This Week.” Mr. Kerry’s 2004 running mate, John Edwards, proposed virtually the same mandate as Mrs. Clinton when he was running for president earlier this year. The former first lady’s health care plan would require all Americans to purchase insurance, while Mr. Obama’s proposal requires only children to be covered.

Mr. Kerry’s remarks represent one of the harshest denunciations of Mrs. Clinton from a policy standpoint in a campaign that has seen relatively few major divides on substantive issues.

Mr. Obama generally has criticized Mrs. Clinton’s plan on its merits rather than on its political feasibility, arguing that the mandate would unfairly penalize people who want insurance but cannot afford it. Mrs. Clinton in turn has accused Mr. Obama of lacking political courage for refusing to include a mandate, which she says is essential to assuring that every American has health care.

“Barack Obama starts with children and works up to a system where, at the back end, you may have a mandate, you will get to universal coverage. But he does it in a way that’s going to give Republicans the opportunity to be able to play at the table,” Mr. Kerry said.

In 2006, the Massachusetts senator proposed his own plan for universal health care that would have mandated coverage for all Americans beginning in 2012.

A spokesman for the Clinton campaign, Howard Wolfson, responded: “We don’t believe that health care legislation that covers every American is a nonstarter; in fact, we believe it is essential to ensuring that every American gets the health care they need.”

Though Republicans have criticized both Mr. Obama’s and Mrs. Clinton’s health care plans, they have not been unanimously opposed to insurance mandates. Proposals endorsed by Governor Schwarzenegger in California and Governor Romney in Massachusetts, both Republicans, have included individual requirements for coverage, and a bill with a mandate introduced in Congress by Senator Wyden of Oregon was cosponsored by eight Republicans. Mrs. Clinton’s proposal “does seem to me to be a plan that has the potential to achieve bipartisan support in the Congress,” a scholar at the Center for American Progress who served as policy director for the Edwards campaign, James Kvaal, said. Health care mandates have “historically been an idea that has some support among Republicans,” he said, because of their element of individual accountability and responsibility.