Now that the American Health Care Act (AHCA) has passed out of the House, it falls into the lap of the Senate, where Republicans are already discussing major changes to the bill.

The plan to replace the Affordable Care Act (ACA) faces a tougher path to passage in the upper chamber. Senate Minority Whip Dick Durbin, D-Illinois, had told HealthExec no Democrat, even those from states President Donald Trump won in the 2016 election, would support the previous incarnation of the bill.

Assuming no Democrats defect this time around, Republicans can only afford two “no” votes among their members, with Vice President Mike Pence breaking a 50-50 tie. Thanks to the budget reconciliation process, Republicans don’t have to meet the higher 60-vote threshold to end a filibuster.

Several Republicans have already expressed doubts about the bill, with Sen. Rob Portman of Ohio saying he won’t support it as written.

“I continue to have concerns that this bill does not do enough to protect Ohio's Medicaid expansion population, especially those who are receiving treatment for heroin and prescription drug abuse,” he said in a statement. “We have an opioid crisis in this country, and I’m going to continue to work with my colleagues on solutions that ensure that those who are impacted by this epidemic can continue to receive treatment.”

Portman is one of four Senate Republicans who signed a letter in March expressing concerns over the AHCA’s cuts to Medicaid, along with Sens. Shelley Moore Capito, R-West Virginia, Cory Gardner, R-Colorado and Lisa Murkowski, R-Alaska. If that faction, or the conservative group made up of Sens. Ted Cruz of Texas, Rand Paul of Kentucky and Mike Lee of Utah, goes against the bill, it would die in the Senate.

Members are also concerned about the bill’s impact on people with pre-existing conditions, with the ability for states to allow insurers to rate customers individually based on their medical history likely to price those with chronic conditions out of the insurance market. It’s one reason why Sen. Dean Heller, R-Nevada, said he can’t support the House legislation.

“I will not support it in its current form in the Senate, and am confident that what the Senate considers and approves will be different from the House bill,” Heller said to the Reno Gazette-Journal. “We cannot pull the rug out from under states like Nevada that expanded Medicaid and we need assurances that people with pre-existing conditions will be protected.”

One possibility is the Senate crafts its own legislation and then relies on a conference committee with the House to work out the differences.

“We will work together carefully to write our own bill,” said Sen. Lamar Alexander, R-Tennessee, according to the Boston Herald. “We’ll make sure we know what our bill costs when we vote on it. ... We will get it right, and then we will vote.”

The healthcare industry, which has been largely united in opposition to the AHCA, is banking on the Senate making big changes or starting over from scratch with more input from major associations.

“We urge the Senate to restart and reset the discussion in a manner that provides coverage to those who need it and ensures that the most vulnerable are not left behind,” said American Hospital Association President and CEO Rick Pollack.