Four Republican senators say they are not ready to vote for the GOP health care bill, putting the measure in jeopardy.

The four are Ted Cruz of Texas, Ron Johnson of Wisconsin, Mike Lee of Utah and Rand Paul of Kentucky. They say in a statement that they are open to negotiation before the full Senate considers the measure.

The four say there are provisions that are an improvement to the current health care system. But they add that the measure fails to accomplish what they have promised to their constituents, "to repeal Obamacare and lower their health care costs."

GOP leaders hope to vote on the bill next week and can only afford two defections from the 52 Senate Republicans.

Medical groups are beginning to weigh in on the Senate Republican health care bill, and they have problems with the proposal.

The American Academy of Pediatrics says the bill would hurt children by scaling back Medicaid. Its president, Dr. Fernando Stein, says the plan was crafted without input from pediatricians and "would tear down" the progress the nation has made by achieving insurance coverage for 95 percent of children.

America's Essential Hospitals, which represents more than 300 safety-net health facilities, says the version the Senate released Thursday "might be worse overall" than the House legislation and might lead to hospitals reducing services or closing.

The Association of American Medical Colleges says the Senate plan would leave millions of people without health coverage, and others with only bare-bones insurance plans.

President Obama also issued a statement condemning the bill on Thursday, urging Republicans to vote against it.