Washington (CNN) The House Ways and Means Committee needed 18 hours to become the first panel to approve the Republicans' Obamacare repeal bill, nearly a full day after debate began and with no end in sight.

Meanwhile Democrats in another committee are continuing to use as many procedural tactics as they can to delay the plan's advancement.

White House and Republican congressional leaders had sought to fast-track the legislation through Congress. Democrats have made clear it wouldn't be easy -- dragging out a the grueling debate in the House Energy and Commerce Committee well into the next day.

In both committee rooms, GOP lawmakers dismissed questions about the bill's cost and claims it would result in millions of Americans losing insurance coverage. Democrats mixed assaults on the legislation with diversionary tactics meant to embarrass Trump -- such as highlighting his refusal to release his tax returns.

The action on Capitol Hill comes against a backdrop of pitched controversy over the legislation, which is posing a crucial first test to Trump's capacity to move his agenda through Congress and as conservative Republicans complain that the initiative does not go far enough in erasing Barack Obama's signature domestic achievement.

Opposition to the Republican health care bill had strengthened Wednesday, as key industry groups that had supported Obamacare said the replacement backed by President Donald Trump could harm vulnerable Americans.

The American Medical Association, which bills itself as the largest organization of doctors in the nation, sent a letter to the two committees, detailing its critique of the bill, known as the American Health Care Act.

"While we agree that there are problems with the ACA that must be addressed, we cannot support the AHCA as drafted because of the expected decline in health insurance coverage and the potential harm it would cause to vulnerable patient populations," the letter stated.

Major hospital organizations, including the American Hospital Association and the Federation of American hospitals have also come out against the bill. The largest seniors organization, the AARP had warned on Tuesday the measure would weaken Medicare and Medicaid. Some conservative medical lobby groups have supported the measure.

White House spokesman Sean Spicer shrugged off the building opposition from the medical establishment.

"We would love to have every group on board," Spicer said, adding, "This isn't about figuring out how many special interests in Washington we can get paid off. It's about making sure that patients get the best deal, that lowers prices and brings back cost."

Trump also showed willingness to jump into the fray, meeting with the leaders of conservative groups that have lined up against the bill.

"This is going to be great. You're going to make it even greater," the President told the group, sources at the White House meeting told CNN. "I'm going to work hard to get it done."

But should the bill eventually fail, Trump outlined a backup plan: Allow Obamcare to fail and let Democrats take the blame, sources said.

Working late

It was a full day -- and night -- for lawmakers on Capitol Hill. Two committees started their markups Wednesday morning at 10:30 a.m.

Eighteen hours in, the Energy and Commerce Committee was still debating the first Democratic amendment -- which was to change the name of the bill. The Ways and Means Committee ended debate on the second of five subtitles in the bill shortly before 11 p.m.

In the Ways and Means meeting room, there were signs that the long day was wearing on lawmakers.

Around 8 p.m., the panel's chairman, Rep. Kevin Brady, leaned back in his chair and put in eye drops.

Later, as restless staffers' side conversations rose to regular volumes rather than whispers, Brady had to call the room to order. Shortly before 4:30 a.m. Thursday, they voted 23-16 to approve their portion of the House legislation.

Aides sat with blankets in their laps or around their shoulders in the overly air-conditioned room. One snuck in a box of Dunkin' Donuts "Munchkins" to share.

The long day unfolded with a series of dry-as-a-bone discussions about health care policy and Democratic procedural measures to make the process as painful as possible for their Republican colleagues.

At one point, the minority forced the clerk of the House Energy and Commerce Committee to read the full bill -- a procedure that took about an hour. Democrats have promised at least 100 amendments, most of which have no chance of passage. For instance, one defeated measure would have blocked health care reform coming into force unless it ensured coverage for every taxpayer and all their dependents.

The GOP legislation unveiled Monday would get rid of Obamacare's individual mandate and put in place refundable tax credits for individuals to purchase health insurance. It also proposes restructuring Medicaid and defunding Planned Parenthood.

The bill looks to preserve some of the more popular elements of Obamacare, including protections for people with pre-existing conditions (though insurers would be allowed to charge higher premiums to individuals whose coverage has lapsed) and letting children stay on their parents' insurance plans until the age of 26.

Democrats want CBO score

Rep. Richard Neal, the top Democrat on the Ways and Means panel, cited the lack of a CBO report as a reason the effort should be postponed. "To consider a bill of this magnitude without a CBO score is not only puzzling and concerning, but also irresponsible," Neal said.

Democratic Rep. Lloyd Doggett said the GOP bill had been kept "as secret as Donald Trump's tax returns."

"It is surprising that you're so determined to hide your panacea," the Texas congressman told his Republican colleagues.

GOP leaders have said that the bill will not be brought to the floor until the CBO scores the legislation.

But while Republican Majority Whip Steve Scalise told the Energy committee that while he is also waiting for a CBO score, he will not let "unelected bureaucrats in Washington" slow down the Republican promise to repeal and replace Obamacare.

One Democratic committee aide said the intent is to "make some Republicans take some really tough votes" that are "revealing" about the GOP's priorities on health care.

Democratic Rep. Brian Higgins highlighted a provision of the bill that provides a tax break for health insurance companies amid criticism that it will simply be passed onto industry executives. Higgins listed the million-dollar salaries enjoyed by CEOs at health care giants like Aetna, Anthem, Cigna and United Healthcare.

It's "morally reprehensible," Higgins said.

Despite the fraught political circumstances, there was a moment of levity at the Energy and Commerce Committee.

Rep. Jan Schakowsky, a Democrat from Illinois, said she was going to blast the Republican plan using President Trump's Twitter vernacular.

"Bad!" she said loudly. "Sad!"

This story has been updated.