Lobbyists representing medical providers and health insurance companies are urging congressional leaders not to repeal Obamacare's individual mandate as part of a tax overhaul being written by Republicans.

"We join together to urge Congress to maintain the individual mandate," the groups, representing doctors, hospitals and insurer wrote in a letter. "There will be serious consequences if Congress simply repeals the mandate while leaving the insurance reforms in place: millions more will be uninsured or face higher premiums, challenging their ability to access the care they need."

The groups sent the letter minutes after GOP Senate leaders told reporters they planned to include repeal of the individual mandate as part of their bill. According to the Congressional Budget Office, doing so would reduce federal spending by $338 billion over the next decade.

CBO also has predicted that repealing the mandate, which requires Americans to buy health insurance or pay a fine, would result in 13 million more people becoming uninsured. Republicans have pushed back against those findings, and CBO is looking into redoing the way it evaluates the individual mandate.

But fewer uninsured would mean a reduction in savings to the federal government. CBO had concluded that reduced spending would result from people dropping Medicaid coverage and from people dropping private coverage that the government helps them pay for, reducing subsidy costs.

In the letter sent Tuesday, medical groups wrote that the mandate was necessary to provide affordable coverage to people with pre-existing illnesses. Proponents have said that some impetus is needed to bring younger, healthier customers into the market so they can help balance out the costs of sicker enrollees.

One of the groups that wrote the letter, America's Health Insurance Plans, previously has supported a waiting period for people who don't sign up for coverage during the designated time, known as open enrollment. Insurers have said that some members under Obamacare are waiting until they are sick to sign up. For doctors and hospitals, the costs of uncompensated care would rise if they were had to provide medical care to more people without health insurance.

"Repealing the individual mandate without a workable alternative will reduce enrollment, further destabilizing an already fragile individual and small group health insurance market on which more than 10 million Americans rely," they wrote. "Under current law, the individual mandate is one of the primary incentives for individuals to enroll in coverage. Eliminating the individual mandate by itself likely will result in a significant increase in premiums, which would in turn substantially increase the number of uninsured Americans."

Other groups that signed the letter include America’s the American Academy of Family Physicians, the American Hospital Association, the American Medical Association, Blue Cross Blue Shield Association, and the Federation of American Hospitals.