WASHINGTON — Two key House committees have approved a Republican proposal to replace the Affordable Care Act, giving the bill its first victories even as the Senate’s top Republican expressed concern about the absence of a projected cost.

The House Ways and Means Committee voted 23 to 16 to advance the American Health Care Act shortly before 4:30 a.m. Thursday after about 18 hours of debate. The House Energy and Commerce Committee voted 31 to 23 to advance the bill at 1:45 p.m. after about 27 hours of debate.

Several hours earlier, Senate Majority Leader Mitch McConnell of Kentucky said lawmakers need to see the Congressional Budget Office’s estimate of how the bill will affect the federal deficit.

“I think we need to know that,” McConnell said at a breakfast sponsored by Politico, adding that the CBO report could be released by Monday.

At the same time, President Trump sought to calm fears about the process.

“Despite what you hear in the press, healthcare is coming along great,” he tweeted midday Thursday. “We are talking to many groups and it will end in a beautiful picture!”

Trump’s tweet came after the two House committees worked at breakneck speed through the night to advance the American Health Care Act. The accelerated pace has drawn criticism from Democrats, who contrasted it with the lengthy deliberations that took place before passage of the Affordable Care Act, as well as from some Republicans.

One of those Republicans, Sen. Tom Cotton of Arkansas, drew swift attention Thursday morning when he tweeted that the House should “start over.”

“House health-care bill can’t pass Senate [without] major changes. To my friends in House: pause, start over. Get it right, don’t get it fast,” Cotton wrote from his political account.

“GOP shouldn’t act like Dems did in [Obamacare],” he continued. “No excuse to release bill Mon night, start voting Wed. With no budget estimate! What matters in long run is better, more affordable health care for Americans, NOT House leaders’ arbitrary legislative calendar.”

Cotton’s tweet added uncertainty to the bill’s fate in the Senate, should it pass the House. Four other Republican senators in states that accepted Medicaid expansion under the ACA, also known as Obamacare, have expressed concerns about changing the way the program is administered.

One of those senators, Rob Portman of Ohio, reiterated his concerns Thursday after meeting with Vice President Pence and Health and Human Services Secretary Tom Price.

“As I have said before, I support making structural improvements to the Medicaid program, but we must provide stability and certainty for individuals and families in Medicaid expansion programs and real flexibility for states,” Portman said in a statement. “I will continue to work with the administration and my colleagues to address these concerns.”

Sen. Angus King, a Maine independent, spoke out against the bill on the Senate floor Wednesday.

The measure also is taking fire on the right from Sens. Ted Cruz, R-Texas, Rand Paul, R-Kentucky, and Mike Lee, R-Utah, who say they want a more complete repeal of the ACA.

Meanwhile, a group with close ties to House Speaker Paul D. Ryan of Wisconsin said it would launch television ads in more than two dozen media markets urging conservatives to support the plan.

“Obamacare put bureaucrats in control. The Republicans’ plan puts patients and doctors in charge. Obamacare stuck families with soaring premiums. The new plan provides more choices and lower costs,” stated the ad from the American Action Network, which will run for two weeks.

Ryan made a new attempt Thursday to explain the full scope of the Republican health plan, explaining in an unusual “TED talk”-style news conference that the pending legislation is only the first step.

The proposal, he explained, is to be followed by Trump administration regulatory actions, as well as additional legislation that will need to win at least some Democratic support in the Senate.

“We as Republican have waited seven years to do this,” Ryan said. “This is the closest we will ever get to repealing and replacing Obamacare. The time is here. The time is now. This is the moment.”

Elsewhere in the House, Ways and Means Committee Chairman Kevin Brady, R-Texas, called his committee’s passage of the health-care proposal early Thursday historic and praised Trump, who has vowed to counter resistance to the measure with his own campaign-style effort.

“Ways and Means Republicans just passed legislation that will help Americans finally have access to affordable health care,” Brady said in a statement after the vote.

“This legislation reflects President Trump’s strong commitment to improving health care for all Americans. I sincerely thank my colleagues for their hard work and commitment to delivering on the President’s promise,” he said.

The passage of the bill through Ways and Means prompted furious responses from Democrats, who said Republicans did not accept a single amendment from the minority party.

Senate Democrats, flanked by people who had benefited from the Affordable Care Act, attacked the Republican later Thursday for allowing the process to take place in the “dead of night.”

Sen. Debbie Stabenow, D-Michigan, who is up for re-election next year in a state that Trump narrowly won, contrasted the speedy handling of the bill to the process that led to the ACA, which involved 14 months of debate and hundreds of hearings.

On the other side of the Capitol, House Minority Leader Nancy Pelosi of California accused Republican leaders of seeking to hide its effects from the public by advancing it quickly.

She highlighted a letter from four Republican committee leaders, including Ryan, sent in June 2009 demanding that the CBO issue an analysis of the Affordable Care Act before their panels voted on the measure. Ryan said this week that it was acceptable and not unusual for two of the same committees to take up the Republican legislation without a CBO analysis.

“The facts are these: We had scores of hearings,” Pelosi said. “Hours of public markup, dozens of Republican amendments accepted.

“They don’t want the American people to see the facts,” she added. “They’re always afraid of the facts.”

House Republican leaders have given little indication that they will make anything but marginal changes to their plan, which would eliminate the requirements that all Americans obtain coverage or pay a tax penalty and that businesses with at least 50 employees provide insurance. The American Health Care Act would replace income-based subsidies with refundable tax credits based on age and income, impose a 30 percent surcharge if people buy a plan after allowing their coverage to lapse and phase out the law’s more generous Medicaid funding over time.

These changes have sparked criticism from powerful health-care stakeholders. The American Medical Association, the American Hospital Association and AARP are among the groups that have rejected the bill.

America’s Health Insurance Plans, the insurance industry’s largest trade association, sent a letter Wednesday saying that although it appreciated some elements of the plan, the proposed changes to Medicaid could produce “unnecessary disruptions in the coverage and care beneficiaries depend on.”

Andrey Ostrovsky, the chief medical officer for Medicaid, took the unusual step of expressing his opposition to the measure Wednesday night.

“Despite political messaging from others at [the Department of Health and Human Services], I align with the experts,” he wrote on Twitter, citing opposition to the bill from the AMA, the American Academy of Pediatrics and the American Academy of Family Physicians.

House Energy and Commerce Committee Chairman Greg Walden, R-Oregon, told reporters Wednesday that critics were exaggerating the proposal’s potential repercussions.

Walden also said it is “sort of shocking” that hospital groups strongly oppose the plan, because it would restore money that the ACA cut from “disproportionate share” payments to hospitals that serve large numbers of uninsured patients.

“There’s a pretty big medical-industrial complex in America,” he added. “And when you touch it, I’ve discovered, it touches back.”

Still, the most imminent threat Republican leaders must contend with comes from the far right. The speaker can lose only 21 Republican votes if the American Health Care Act is to pass, and opponents are promising to use that leverage to force changes to the bill.

Pence met with two Freedom Caucus leaders Tuesday, and that same day, White House Budget Director Mick Mulvaney – a former caucus member – spent more than an hour at a meeting of the group. Its members have been invited to visit the White House next Tuesday.

On Wednesday night, Trump met with leaders from Americans for Prosperity, the Club for Growth, FreedomWorks, the Heritage Foundation and its political arm, Heritage Action, and the Tea Party Patriots.

Amy Goldstein, Carolyn Y. Johnson, Sean Sullivan and David Weigel contributed to this report.