President Donald Trump doesn't have the votes to pass his health care bill, the chairman of the House Freedom Caucus said Wednesday, but negotiations are underway.

A spokesperson for Rep. Mark Meadows, R-North Carolina, said the conservative group is "cautiously optimistic" that it will get what it wants after Meadows, the caucus' chairman, and other members met with Trump and Vice President Mike Pence at the White House on Wednesday.

But they're all trekking back again Thursday — the same day the House is expected to vote on the bill.

"We need changes to the underlying bill before we vote on it in the House. ... There's not enough votes to pass it tomorrow," Meadows told reporters Wednesday.

The Freedom Caucus has staunchly opposed the current Republican plan, called the American Health Care Act, in part because it says the bill would enshrine Medicaid and create a new entitlement program.

Members are lobbying the White House for last-minute changes to regulations that would be imposed on insurance companies, since the AHCA kept many that were first imposed by the Affordable Care Act. Conservatives consider the regulations a symptom of heavy-handed government, including the requirement that insurance companies cover certain things such as maternity and preventative care.

Related: These Republicans Could Doom Their Party's Health Care Bill

House leadership has been reluctant to incorporate their demands, saying the changes would violate rules that allow the Senate to pass the measure with a simple majority — just 51 votes — instead of the usual 60 votes.

But hours before the bill is supposed to come to the floor in the House, leadership seemed to be relaxing the parameters. A senior Republican aide said members have received updated guidance from the Senate that while the changes would likely be challenged, the challenge would not necessarily kill the bill.

The changes would be a major win for the House conservatives, who have been holding out support for the bill and threatening to doom its passage. The task of assembling party-line votes for the AHCA has proven difficult, given that different members with the Republican conference oppose the bill for wildly different reasons.

House Speaker Paul Ryan, R-Wisconsin, was spending a large part of his evening meeting with undecided members or those who leadership think are persuadable. He was focusing on the more moderate members of the House and leaving the conservatives to the White House.

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At least 28 Republicans, according to an NBC News tally, are planning to vote no or are leaning in that direction. The measure needs the support of 215 members — a majority of the 434 who are expected to vote — to pass the measure.

The drama of corralling an unruly Republican conference comes one day after Trump told Republicans that they'd lose their seats if they vote against the bill.

But influential outside groups have staked out their positions. Major conservative groups — including Freedom Partners and Americans for Prosperity, which are backed by Charles and David Koch; Heritage Action; FreedomWorks; and Club for Growth — are urging Republicans to vote against it. The groups back primaries against Republicans who don't align with their positions.

The Koch-backed groups said Wednesday night that they are setting up a "seven-figure" fund to support "principled lawmakers" who vote against the measure, which they say isn't a full repeal of Obamacare.

"Republicans in Congress promised a full repeal of Obamacare, but the current plan falls far short," AFP Chief Government Affairs Officer Brent Gardner said. "We simply cannot support this bill and commit to standing with champions in the House who also oppose the continuation of Obamacare."

On the other side, the U.S. Chamber of Commerce, which supports the measure, is also keeping score of the members who vote against it.

It's not only Republican conservatives who oppose the bill. About a dozen moderates opposite it, as well, but for different reasons. They are concerned that the tax credits aren't generous enough and that the Medicaid changes would kick too many people off the program for low-income Americans.

Despite cajoling, moderate Rep. Dan Donovan, R-New York, announced his decision in a local news opinion piece Wednesday afternoon.

"My goal for this whole process was to help the people that it harmed without harming the people that it helped," Donovan told reporters.

According to a nonpartisan Congressional Budget Office assessment of the Republican bill, 24 million people could lose health insurance by the end of the decade. Premiums for people over 50 who purchase insurance in the individual market could see their premiums rise from $1,200 to $14,000 a year.

A senior Republican aide told NBC News that there are no plans to delay the vote beyond Thursday.

Former Vice President Joe Biden, standing with House Democrats on the East Steps of the Capitol on Wednesday to commemorate the seventh anniversary of the Affordable Care Act's enactment, admitted that changes to Obamacare are needed, but he said a repeal like the one included in the AHCA would be dangerous.

"This bill was about peace of mind! Peace of mind for Americans to finally, finally, finally, finally be able to lie there and know that, God forbid, something happens like it's happened to an awful lot of us, that I'm not going to lose my house, I'm not going to lose everything, I'm not going to leave my family in distress," Biden said.

On their own, Democrats have little power to block Republican efforts in the House, but they have roundly denounced the AHCA as a generous tax cut for the wealthy masquerading as health care reform. The measure would repeal all Obamacare taxes, giving people making more than $200,000 a tax cut of 4.7 percent.

The nonpartisan Congressional Budget Office estimated that under the Republican plan, premiums would decrease by just 10 percent over a decade.

Some also oppose the size of the tax credits to help people purchase insurance and a provision that would require insurance companies to provide patients with minimum coverage options such as maternity care, also known as the Essential Benefits Package.

Rep. Jim Jordan, R-Ohio, said Wednesday that he's still opposed to the Republican plan.

"Our job is to do just what Mark Meadows said — to do what we told the American people we were going to do when they gave us the privilege to serve," Jordan said Wednesday on MSNBC's "Morning Joe."

"And this bill doesn't do that. This bill does not repeal Obamacare," he said. "And that fundamentally is why we are opposed to it. And unless it changes, I do not see the votes there to pass this legislation."

The only place the bill can change is in the House Rules Committee, which is currently meeting. It is the final step in the process before the measure goes to the House floor.

Changes are being proposed, including a full repeal of Obamacare like Jordan suggested, but the Rules Committee is stacked with Republicans close to leadership, and any changes not mandated by leadership are likely to fail.