COLLEGE PARK, Md.

President Barack Obama called reinventing health care a “defining struggle of this generation” on Thursday, even as several Democrats criticized some of the fine print in a Senate proposal the president has praised.

Obama told about 15,000 college students at a campaign-style rally that Congress must resist scare tactics and false accusations to remake the nation's health care system.

The fight will be difficult, he said, and resistance that started surfacing Thursday to a key provision in Sen. Max Baucus' bill illustrated that. Several Democrats expressed concerns about a new tax proposed by Baucus, a Montana Democrat who chairs the Finance Committee. It would apply to high-value health insurance policies, but some say it would hit too many middle-class workers.

Obama, seemingly energized by the wildly cheering students at the University of Maryland, said forces are lining up in favor of a major health care bill, even as “special interests” oppose it. He said an “unprecedented coalition” of hospitals, doctors, nurses and drug makers support the effort. Some of the most enthusiastic backers, he said, “are the very medical professionals who have firsthand knowledge” of how badly the current system operates.

“When I sign this bill, it will be against the law for insurance companies to drop your coverage when you get sick,” the president said.

Obama aimed many of his remarks directly at young adults, a group that activists would like to see more involved in the push for a health care overhaul.

He again called for a public insurance option, which most congressional Republicans, and some Democrats, oppose. It would not amount to “a government takeover of health care,” he said. Obama stopped short of insisting on such a plan.