WASHINGTON (AP) — A Republican bill easing how experimental drugs are provided to people with terminal illnesses headed toward a House vote Tuesday, but opposition by top Democrats and scores of patients' groups left its fate uncertain.

The "Right to Try" legislation, backed by President Donald Trump and Vice President Mike Pence, gives fresh hope to patients with fatal diseases and erects safeguards to protect them, Republicans said.

Democrats said it would achieve little since the federal Food and Drug Administration already approves 99 percent of the 1,000 requests it receives annually for an existing program that lets patients use unproven treatments. They also said the proposal would create risks by rolling back the FDA's oversight role in the process.

The FDA would no longer need to sign off if a doctor and pharmaceutical maker agree that a patient with no other options should try an experimental treatment.

The agency would have to be notified after treatment begins, manufacturers would have to report if problems arose and drug makers and doctors would be broadly shielded from legal liability. Manufacturers would still not be required to provide their pharmaceuticals.

Drew Hammill, spokesman for House Minority Leader Nancy Pelosi, D-Calif., said Tuesday she will "follow the lead of numerous patient and disease groups in opposing this legislation."

Democratic support was crucial because Republicans were debating the measure under rules — usually reserved for uncontroversial bills — requiring a two-thirds majority for passage. That meant majority Republicans would likely need at least 50 Democrats, or nearly 1 in 4.

More than 70 patients and research groups sent a letter of opposition Monday to House leaders saying the bill "would not increase access to promising therapies" and "is still less safe" that the FDA's existing process.

Eliminating required FDA approval for use of the drugs erases "some really important protections for patients" because of the agency's knowledge, said Dr. Richard Schilsky, chief medical officer for the American Society of Clinical Oncology, which signed the letter.

House Speaker Paul Ryan, R-Wis., tweeted that the measure would "empower patients and their families," adding, "For those who are faced with terminal conditions, they deserve the #RightToTry."

The Senate passed a similar bill in August.

Critics said the measure misleadingly suggests that the FDA is the bottleneck to providing the experimental treatments. They say the more common hurdle is manufacturers, which often prefer to use their limited quantities of early-stage drugs for clinical trials needed to gain final FDA approval for wide-spread sales or worry that a setback could damage the product's marketability.

Scott Gottlieb, FDA commissioner and a Trump appointee, told Congress in October that 70 percent of the experimental drugs his agency currently lets seriously ill patients use are never approved.

"The vast majority of people" granted access to unproven treatments "are using a drug that doesn't work," he said.