Republican leaders launched a full-court press Tuesday to rally support for the party's newly revealed health care plan as criticism mounted from rank-and-file members and prospects for the legislation appeared uncertain at best.

Vice President Mike Pence traveled to Capitol Hill to sell the GOP leadership's plan to repeal and replace the Affordable Care Act and met with those lobbing the heaviest criticism at the bill. President Trump met with Republican leaders at the White House to discuss the bill's passage.

Meanwhile, Health and Human Services Secretary Tom Price spoke at length during the daily briefing at the White House. And the chairmen of the relevant House committees, Reps. Kevin Brady and Greg Walden, who wrote the bill, held two news conferences (including one with House Speaker Paul Ryan) as they blanketed television news shows to sell the plan.

The push came after many Republicans balked at the details of the bill when it was released late Monday. Some called the proposal "Obamcare Lite" while others questioned whether it would provide adequate coverage, demonstrating the challenges facing GOP leadership in trying to pass the legislation.

Members of the House Freedom Caucus and two conservative Republican senators — Rand Paul of Kentucky and Mike Lee of Utah — held a press conference outside the Capitol Tuesday afternoon to press their case against the bill.

"There's one thing that has united Republicans in 2010 when we won the House, in 2014 when we won the Senate, and in 2016 when we won the White House. This doesn't divide Republicans, this brings us together, and that is complete repeal, clean repeal," Paul said.

We are united on repeal, but we are divided on replacement," Paul said.

President Donald Trump singled Paul out in a tweet Tuesday evening.

I feel sure that my friend @RandPaul will come along with the new and great health care program because he knows Obamacare is a disaster! — Donald J. Trump (@realDonaldTrump) March 8, 2017

While the topic is complicated, the math for passing the bill is simple. Republican leaders need to find 218 votes in the House and 50 in the Senate (with Pence available as a tie-breaker). With little to no support expected from Democrats, GOP leaders can't afford many defections.

Tuesday's leadership push also comes as conservative groups with deep pockets and millions of activist members have signaled their opposition to the replacement bill, putting more pressure on jittery Republicans.

Related: New GOP Health Care Bill Has Host of Critics

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The complaints coming from Republicans cut across the party's ideological fissures. Small-government conservatives have criticized it as another version of Obamacare. They say that the age-based tax credits to help people purchase health insurance are no different from the income-based subsidies currently offered through the Affordable Care Act.

“This is not the Obamacare repeal bill we’ve been waiting for. It is a missed opportunity and a step in the wrong direction,” said Sen. Mike Lee, R-Utah, adding that it was a "missed opportunity" to go big and bold.

What also concerns these conservatives, many of whom are members of the tea-party minded Freedom Caucus, is that it's not a full repeal of the Affordable Care Act, noting that it keeps in place the Medicaid expansion until 2020 and doesn't actually get rid of the Obamacare exchanges. And they see little evidence that it will actually lower health care costs.

"Our plea: Repeal and replace with a patient-centered, doctor-centered plan," said Rep, Jim Jordan, R-Ohio, and member of the Freedom Caucus, which has a membership of about 40 Republicans. Some of these conservative members have authored their own replacement bill, and they want it introduced alongside the House leadership bill.

On the opposite end of the Republican spectrum, several members of the Senate have expressed concerns about the the changes to the Medicaid expansion, which after 2020 will change from an income based requirement to a limit based on population. They say that it is too drastic of a roll-back of the Medicaid expansion.

Four Republicans in the Senate, including Rob Portman of Ohio, who comes from a state that expanded Medicaid, and Shelly Moore Capito of West Virginia, another state with a large Medicaid population, have voiced their concerns about that part of the plan.

Also challenging passage of the AHCA is the one-year defunding of Planned Parenthood. Two Republican senators, Lisa Murkowski of Alaska and Susan Collins of Maine, have expressed reservations about that feature. “I don’t think the Planned Parenthood defund should be in the ACA bill that we’re dealing with,” said Murkowski.

But Republican leaders are indicating that major changes to the bill are unlikely, even though President Donald Trump tweeted Tuesday morning that the health care bill is out “for review and negotiation.” But it’s unlikely that any negotiation for changes will occur.

Sen. John Thune, R-S.D. and a member of Republican leadership, said that the Republican opposition is “leverage.”

“Everybody right now is trying to leverage their position … but when push comes to shove, it’s going to either be a vote for a status quo or a vote to repeal this and to a better way,” Thune said. “We have a difference of opinion right now but I think before this is all said and done consensus will emerge.”

Related: Republicans Unveil Obamacare Replacement Bill

The House Energy and Commerce Committee and the Ways and Means Committee will debate the bill Wednesday, likely the only window for any altering the bill, and that would likely amount to small tinkering, not structural changes.

Chairman Brady, head of the House Energy and Commerce Committee, defended the bill as taking conservative principles but he also put down a marker for critical Republicans.

“As Republicans we have a choice: We can act now or we can keep fiddling around and squander the opportunity to repeal Obamacare and begin a new chapter,” Brady said.

But some Republicans are holding out hope that President Trump will be on their side.

"Everyone wonders. We're all waiting to hear that," said Rep. Dave Brat, R-Virginia, when asked if Trump is on the same page as Speaker Ryan and and the authors of the bill. "I haven't heard him come down in thunder in approval on this."

The president called it a "wonderful" bill in a Tuesday morning Tweet, but also noted it was out for "review and negotiation.

Our wonderful new Healthcare Bill is now out for review and negotiation. ObamaCare is a complete and total disaster - is imploding fast! — Donald J. Trump (@realDonaldTrump) March 7, 2017

Republicans are also taking a beating for releasing a bill without an official score on how much the bill will cost. They say the non-partisan Congressional Budget Office will release a budget outlook before it comes before the House floor.

"I would want to know the score, you know, what is the coverage and what is the cost, absolutely," said Sen. Bill Cassidy, R-Louisiana. "I'm trying to be diplomatic."

Influential conservative groups have also sharply criticized the bill. The Charles and David Koch-backed groups Americans for Prosperity and Freedom Partners came out against it because of the tax credits but also because of the penalty for people who don’t purchase insurance. The Republican plan attempts to incentivize continuous health care coverage by allowing health insurance companies to charge an extra 30 percent on premiums.

Hilgemann calls it a mandate for coverage but one where the penalty is reaped by the insurance companies instead of the government, like in Obamacare.

“We’re really focused on telling the story of decades’ worth of work that AFP has done to see real transformative health care reform in this country. What we saw come out of the House last night just falls short of that,” said Luke Hilgemann, CEO of Americans for Prosperity. “This law is broken we need a better solution and what we’ve seen out of Washington so far is not that.”

AFP has been organizing around its opposition to Obamacare for years, using that as its top motivating factor in getting conservatives involved in its organization and to the polls. Their opposition could mobilize its members to pressure Republican lawmakers, further complicating the law’s passage.

And Club for Growth and the Heritage Foundation, two other conservative groups with influence among Republican members of Congress, also came out strongly against it. President David McIntosh called it a “warmed-over substitute for government-run health care.”

“The problems with this bill are not just what’s in it, but also what’s missing: namely, the critical free-market solution of selling health insurance across state lines,” McIntosh, who dubbed the measure “RyandCare,” said. “Republicans should be offering a full and immediate repeal of Obamacare’s taxes, regulations, and mandates, an end to the Medicaid expansion, and inclusion of free-market reforms, like interstate competition.”