The House bill to repeal and replace ObamaCare is facing growing opposition from Senate Republicans. 

The plan, titled the American Health Care Act, is taking fire from both conservative and moderate factions of the Senate GOP caucus — underscoring the legislation's perilous path in the upper chamber.

Republicans hold 52 seats, a narrow majority, in the Senate. So GOP leadership can only afford to lose two Republican votes if all Democrats and Independents oppose the measure, which would then require Vice President Pence to break the tie.

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Three days after the release of the House bill, GOP senators are signaling that it faces a nearly impossible climb in its current form.

Sen. Ted CruzRafael (Ted) Edward CruzIs this any way for NASA to build a lunar lander? GOP strategist predicts Biden will win nomination, cites fundraising strength 3 real problems Republicans need to address to win in 2020 MORE (R-Texas) expressed skepticism Wednesday that the measure would be able to win enough support among Senate Republicans to pass.

"The House bill is a beginning, but the House bill as drafted, I do not believe, would pass the United States Senate," the conservative firebrand told reporters.

He said the House bill doesn't do enough to lower the cost of insurance and pointed out "significant challenges" stemming from its provision to end Medicaid expansion.

The House bill dramatically restructures the Medicaid program overall by capping federal payments, and in 2020 it would cut off the extra federal dollars ObamaCare sent to states to cover more low-income residents. 

Cruz made his comments after Sen. Rand PaulRandal (Rand) Howard PaulGraham promises ObamaCare repeal if Trump, Republicans win in 2020 Conservatives buck Trump over worries of 'socialist' drug pricing Rand Paul to 'limit' August activities due to health MORE (R-Ky.) told reporters that the House bill, which he has dubbed “ObamaCare lite," will be “dead on arrival” in the upper chamber.

“I think the White House, the administration and the president understand that there's enough conservatives that they can't pass ObamaCare lite,” Paul told CNN’s “New Day” on Wednesday.

The Kentucky Republican is teaming up with members of the House Freedom Caucus to roll out an alternative repeal bill that mirrors 2015 legislation supported by conservatives.

But that measure, which goes further than the current proposal, would likely alienate the moderate GOP senators who are already wary of the House legislation.

Sen. Susan CollinsSusan Margaret CollinsSusan Collins challenger hit with ethics complaints over reimbursements Overnight Health Care: Insurance lobby chief calls Biden, Sanders health plans 'similarly bad' | Trump officials appeal drug price disclosure ruling | Study finds 1 in 7 people ration diabetes medicine due to cost Collins downplays 2020 threat: 'Confident' reelection would go well if she runs MORE (R-Maine) said Wednesday that she’s “not crazy” about the current repeal measure.

"I do not think it will be well received in the Senate,” she told Yahoo News.

Collins is one of a handful of centrist Republicans who are concerned about what happens to ObamaCare’s Medicaid expansion.

The Maine Republican, who voted against the 2015 repeal bill, said she has "concerns" about the Medicaid language, as well as the provision to cut off Planned Parenthood funding.

Any combination of the roughly eight senators who’ve expressed reluctance about the House bill is more than enough to defeat it.

But House Republicans are moving forward despite the looming showdown with the Senate.

Two House committees spent hours marking up their portions of the bill on Wednesday, with Republicans rejecting a slew of amendments from Democrats but offering no changes of their own. 

Rep. Greg Walden (R-Ore.) defended the process, saying, "We've got an open legislative process under way right now."

But multiple GOP senators are signaling that they feel the House is moving too quickly, with Collins and Sen. Tom CottonThomas (Tom) Bryant CottonGOP senator says he suggested Greenland purchase to Trump, met with Danish ambassador It's time to empower military families with education freedom Cotton warns China: Crackdown on Hong Kong would be 'grave miscalculation' MORE (R-Ark.) as the latest to argue that lawmakers should slow down.

“I think we’re moving a little bit too quickly on healthcare reform,” Cotton told MSNBC’s “Morning Joe.” “This is a big issue.”

Cotton noted he was reviewing the bill and said he "would much sooner get healthcare reform right than get it fast."

Top Republicans in both chambers are arguing their colleagues will eventually come around. They say GOP lawmakers have just two choices: support the House bill or be seen as supporting ObamaCare, which they’ve pledged for years to repeal.

“Here, I guess, is my main point. As Republicans, we have a choice: we can act now or we can keep fiddling around and squander this opportunity to repeal ObamaCare and begin a new chapter for the American people,” Rep. Kevin BradyKevin Patrick BradyRepublicans' rendezvous with reality — their plan is to cut Social Security The Social Security 2100 Act is critical for millennials and small business owners House panel releases documents of presidential tax return request before Trump MORE (R-Texas) told reporters this week. 

Sen. John ThuneJohn Randolph ThuneSchumer blasts 'red flag' gun legislation as 'ineffective cop out' Lawmakers jump-start talks on privacy bill Trump border fight throws curveball into shutdown prospects MORE (S.D.) — the No. 3 Senate Republican — echoed Brady this week at a leadership press conference, saying, “It’s going to be either a vote for the status quo or a vote to repeal this and to move to a better way ... and I think that’s the way it’s going to be framed.”

The hardline comes even as the White House launches an all-out charm offensive to sell the bill. Pence is making the rounds on Capitol Hill, while key conservatives are heading to the White House.

After meeting with Cruz this week, members of the House Freedom Caucus are expected to huddle with Trump.

A caucus source told The Hill that members of the group, which panned the White House-backed proposal, have been invited to the White House next week for a pizza dinner and policy discussion at the bowling alley.

And President Trump appears to be signaling that he’s willing to make a deal, tweeting that the House legislation is “out for review and negotiation.”

It’s unclear if Senate Republicans will amend the bill once it passes the House. Senate Majority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellDavid Axelrod after Ginsburg cancer treatment: Supreme Court vacancy could 'tear this country apart' Pelosi asks Democrats for 'leverage' on impeachment Democrats press FBI, DHS on response to white supremacist violence MORE (R-Ky.) said Tuesday that he hopes to bring up the House version, urging his GOP colleagues to start reviewing it.

Asked about the potential for changes, Sen. John CornynJohn CornynThe Hill's Morning Report - Trump hews to NRA on guns and eyes lower taxes The Hill's Morning Report - Trump on defense over economic jitters Democrats keen to take on Cornyn despite formidable challenges MORE (R-Texas) told The Hill, “We'll see what the House passes."

But multiple Senate Republicans said they expect the lower chamber’s measure will undergo changes before it reaches them.

"It is going to continue to develop,” Sen. Marco RubioMarco Antonio RubioGOP group calls on Republican senators to stand up to McConnell on election security in new ads What the gun safety debate says about Washington Trump moves forward with F-16 sale to Taiwan opposed by China MORE (R-Fla.) told reporters. “There's no way the bill that is before us now will be what it looks like when it passes the House."

Cruz said Wednesday he’s in “active negotiations” with GOP lawmakers, as well the administration, on potential changes to the bill. He and his wife are slated to have dinner at the White House on Wednesday night.

If senators change the legislation once it reaches the Senate, it would have to bounce back to the House for a second vote before being sent to Trump's desk.

Collins stressed that she considered the House bill a “work in progress.”

"The bill that was released this week is far better than the bill that we were briefed on the week before,” she said. “So, who knows maybe it’ll eventually get better. And thus will be better received in the Senate.”

Peter Sullivan contributed.