WASHINGTON — Senate Republicans face increasing pressure to rescue health insurance markets and protect coverage for millions of Americans amid growing fears the Trump administration is going let the markets collapse.

In recent days, leading hospitals, physician groups, health insurers and the U.S. Chamber of Commerce have pleaded with the Senate to step in, effectively going around the White House.

“Congress must take action now,” the groups warned in a letter to GOP and Democratic Senate leaders. “At this point, only congressional action can help consumers.”

It remains to be seen whether the GOP-controlled majority — which has been considering more sweeping legislation to roll back the Affordable Care Act — will be willing to shift gears and coalesce around a more short-term fix.

Senate Majority Leader Mitch McConnell, R-Ky., is laboring to unite GOP senators around a broader bill.

But Senate Republicans may be forced to turn to a more urgent task as growing numbers of insurers threaten to raise premiums or exit markets around the country.

Polls suggest that the majority of Americans now hold Republicans responsible for the fate of the nation’s health care markets, potentially imperiling GOP lawmakers who are up for re-election next year.

“There clearly has to be sort of a short-term solution,” Sen. John Thune of South Dakota, the No. 3 Republican, said Tuesday. “We have to do that. There’s got to be certainty in the marketplace. There has to be some certainty with the insurers.”

According to several lawmakers, Senate Republicans have begun to discuss a “transition plan” to stabilize markets over the next several years.

Trump and many GOP lawmakers, including McConnell, have blamed the proposed premium hikes and market exits on weaknesses in the Affordable Care Act.

But a growing number of health insurers and state regulators, while noting problems with the current law, say the Trump administration is the primary cause of current turmoil.

The president has sent mixed signals about enforcing penalties on people who don’t buy health insurance. The penalty is seen as critical to inducing younger, healthier people to get coverage.

And Trump has repeatedly threatened to withhold federal aid that helps millions of low-income Americans afford their deductibles and co-pays.

On Monday, America’s Health Insurance Plans, the industry’s Washington-based lobbying arm, called the Trump administration’s refusal to commit to paying the aid “the single most destabilizing factor in the individual market.”

The warnings have been echoed by leading hospitals and physicians, who increasingly worry about the erosion of insurance coverage.

“Patients who don’t have insurance live sicker and die younger,” said Dr. Andrew Gurman, president of the American Medical Association, which signed on to the recent letter to Senate leaders. “We have to provide for these people to have insurance.”

Without the aid, insurers almost certainly will raise rates, which would most affect middle- and upper-income consumers who have been most critical of the current law and have fueled the GOP repeal campaign.

Lower-income Americans, by contrast, likely would be more insulated from the premium hikes because they get other assistance through Obamacare.

With the White House unwilling to act, many are now turning to Congress, which could vote to appropriate the aid, thus bypassing the Trump administration.

In addition to paying the aid — known as Cost Sharing Reductions, or CSRs — for this year and next, Congress also could provide additional funding to protect insurers from costly and unexpected medical claims.

Such funding, which is common in other insurance markets such as Medicare’s Part D prescription drug program, helps to lower risk for insurers and thereby decrease pressure on them to raise premiums.

But McConnell and other senior GOP leaders have thus far indicated no public interest in such a move. And conservative groups that drive much of the congressional Republican agenda continue to push hard for a sweeping repeal.

noam.levey@latimes.com