The health insurance company targeted by the Obama administration and a top Senate Democrat properly followed guidance dating back to the Clinton administration and should not be punished, Republicans said Thursday. 

House and Senate Republicans have put up a wall of resistance against the decision by the Centers for Medicare and Medicaid Services to launch a probe into Humana for sending to its members what Democratic Sen. Max Baucus called "false" information about health care reform legislation. 

Senate Republicans on Thursday threatened to block Obama's health care-related appointments if the decision is not reversed, and House Republicans called for a hearing on what they described as a politically motivated "gag order." 

"It's an astonishing overreach," Senate Minority Leader Mitch McConnell, R-Ky., told FOX News on Thursday.

CMS began the probe at the request of Baucus, D-Mont., the chairman of the Senate Finance Committee, over a mailer Baucus claims misled seniors about proposed changes to Medicare. CMS also said it was investigating whether Humana inappropriately used lists of Medicare participants. 

Humana, one of the largest private carriers serving seniors under the Medicare Advantage program, focused its mailer on the potential for cuts to service. 

Republicans said Humana's claim was accurate in the first place. But as proof its behavior was above board, they held up a Clinton-era letter from the Department of Health and Human Services offering guidance on mailers sent by insurers to customers. 

The 1997 letter, written by Center for Health Plans and Providers Director Bruce Fried to a law firm, addressed the question of whether HMOs could tell members about proposed legislation and urge them to express their opinions. The letter concluded that restrictions could violate free speech laws. 

"While it may be difficult for a reviewer to ascertain whether or not the information about legislation, for example contained in a member's newsletter issued by an HMO, is accurate and without a slant or unrevealed self-interest, we believe that prohibiting such information would violate basic freedom of speech and other constitutional rights of the Medicare beneficiary as a citizen," the letter reads. 

The letter noted that information about members' and HMOs' rights and responsibilities should not be "misrepresented," but the general requirement for such mailers would be an attached disclaimer saying the information has not been reviewed for "accuracy or misrepresentation." 

Humana attached a nearly identical disclaimer in its mailer. 

Rep. Dave Camp, R-Mich., ranking member of the House Ways and Means Committee, said the letter is proof that CMS was in conflict with its own policy. 

"This actually reverses a Clinton administration policy that said, actually, seniors have a right to know how their benefits may be affected," Camp told FOX News.

McConnell and other top Senate Republicans cited the same evidence in their letter to Health and Human Services Secretary Kathleen Sebelius Thursday in which they demanded that the "gag order" be lifted. 

"Now, the Obama administration has reversed this longstanding HHS decision -- in the midst of a critical debate about the future of health care services in our country -- to shut down communication between private companies and America's seniors on an issue that has a direct impact on their health care," the letter said. 

Republicans said the administration essentially was punishing Humana for questioning the plan and firing a warning shot at any other companies that might be thinking of doing the same. 

The Department of Health and Human Services on Monday not only targeted Humana, but sent out a broad directive to all Medicare Advantage participants, telling them to "immediately discontinue all such mailings" and remove any such material from their Web sites. 

Robert Zirkelbach, spokesman for America's Health Insurance Plans, said the order went out to about 200 companies Monday night, just as the Senate Finance Committee was about to start debate on its version of health care reform. 

"This is an effort to stifle any dissent," he said. 

"They are silencing opposition to the president's Medicare cuts," said Camp spokesman Sage Eastman. 

Camp and the other minority members of the House Ways and Means Committee wrote a letter Thursday to Chairman Charles Rangel, D-N.Y., urging a hearing in order to "investigate the unusual and potentially politically motivated decision by CMS to eliminate the flow of factual information from private health plans to their enrollees." 

Humana has donated heavily to both Democratic and Republican lawmakers over the last decade, including McConnell and Camp but also Baucus. Baucus is more heavily funded by health insurance companies like Aetna and Blue Cross/Blue Shield. 

In its original mailer, Humana expressed concern about proposals to cut Medicare and Medicaid spending by about $500 billion over 10 years, including payments to Medicare Advantage plans worth about $125 billion. 

"While these programs need to be made more efficient, if the proposed funding cut levels become law, millions of seniors and disabled individuals could lose many of the important benefits and services that make Medicare Advantage health plans so valuable," the mailer reads. 

It urged seniors to sign up with Humana for regular updates on the legislation and encouraged them to contact their lawmakers in Washington. 

Congressional Budget Office Director Douglas Elmendorf on Thursday agreed that seniors in the Medicare Advantage plans could see reduced benefits under Baucus' legislation. 

But Baucus said the proposed bill would not cut benefits. 

"I'm not going to let insurance company profits stand in the way of improving Medicare for seniors," he said in a statement, calling efforts to mislead seniors "wholly unacceptable." 

Analysts say the Medicare Advantage plans are being paid too much -- about 14 percent more than it costs to care for seniors in traditional Medicare. 

Democrats continued to assert that the company had made a false claim and that Republicans were again demonstrating their affinity for the insurance industry. 

Baucus said the mailer could be a violation of federal regulations. 

But AARP, which also helps administer Medicare plans in conjunction with United Healthcare, has weighed in on that part of the health care debate as well -- only on the other side. 

The AARP continues to feature ads on an affiliated Web site defending the Medicare changes. One ad blasts critics for spreading "myths and scare tactics," and claims the reforms will not "hurt" Medicare but "actually strengthen it by eliminating billions of dollars in waste and lowering drug prices." Another AARP article declares, "Controlling the rising costs of Medicare doesn't mean cutting benefits." 

Eastman said AARP is not being held to the same standard as Humana. 

"If you're going to silence the critics you need to silence the proponents too," he said. "This clearly smacks of politics." 

AARP Executive Vice President Nancy LeaMond issued a statement dismissing such criticism. 

"Any effort to derail AARP's commitment to reform will not succeed. Similar to 'death panels' and other scare tactics, this latest effort is a misguided attempt to talk about anything other than the health care reform this country needs," LeaMond said. 

FOXNews.com's Judson Berger and The Associated Press contributed to this report.