The White House and its allies scrambled Thursday to quash a growing liberal assault on a much-compromised health care proposal, hoping to keep President Barack Obama's top domestic priority from being crushed between the political left and right.

In Senate speeches, TV appearances, blogs and other outlets, Obama's supporters said the latest attacks are exaggerated and troubling, because Senate Democratic leaders can't spare a single vote in trying to overcome fierce GOP opposition.

But some prominent liberals, led by former presidential candidate Howard Dean, say the Senate bill is so diluted that it's worse than nothing at all. Powerful labor unions were equally disenchanted but urged lawmakers to press on, hoping to improve the bill in House-Senate negotiations.

"Any measure that expands private insurers' monopoly over health care and transfers millions of taxpayer dollars to private corporations is not real health care reform," Dean, a doctor and former Vermont governor and national Democratic Party chairman, said in an op-ed column in the Washington Post.

Top White House adviser David Axelrod disputed Dean's claims and urged party activists to embrace an important if imperfect bill.

"We're on the doorstep of doing something really meaningful," Axelrod said in an interview. No one is entirely satisfied with the bill, he said, but it includes long-sought health insurance regulations and other items too important to lose.

Former president Bill Clinton said in a statement it would be "a colossal blunder" to let the measure die. "America can't afford to let the perfect be the enemy of the good," he said. "Inaction leads to more uninsured Americans, more families struggling to keep up with skyrocketing premiums," and other woes.

Liberal disenchantment with Obama has been simmering for weeks, as he ordered 30,000 more troops to Afghanistan and declined to fight for a government-run health insurance option. It reached a boil this month when Senate leaders dropped the proposed "public option" from the health bill, and then jettisoned a bid to open Medicare to people 55 and older.

Dean, whose relationship with Obama is chilly, previously had endorsed the Senate bill, and his turnabout startled the political world. Many liberals still see him as a champion, and his brother, Jim, heads Democracy for America, a liberal group opposing the Senate bill in its current form.

Obama replaced Dean as Democratic Party chairman with Virginia Gov. Tim Kaine and, in a snub, didn't invite Dean to the announcement ceremony. Dean also signaled interest in a top post in the Obama administration, but received none.