McConnell can only afford two defections and still pass his Obamacare repeal legislation, but Republicans only saw McConnell’s draft bill for the first time Thursday. With both moderates and conservatives expressing deep concern about McConnell’s "discussion draft," he faces a narrow path to passage.

Defeat may scuttle plans to replace Obamacare for the foreseeable future.

“It will hurt the Republicans if they fumble on the issue that has been their signature issue," said Julian Zelizer, an historian at Princeton University. "That won’t look good if they can’t do this. This is a high-stakes moment.”

Related: Four GOP senators spurn health plan aimed at unifying party

And for McConnell, it will show whether he can move beyond his reputation as an obstructionist and deliver on his party’s biggest priorities.

The health care measure could dramatically affect many Americans’ health and financial security while also posing challenges to state governments facing proposed cuts in Medicaid coverage for low-income residents.

Related: How killing Obamacare might save some of Obamacare, for a while

McConnell’s challenge is similar to the one Speaker Paul Ryan faced when the House passed its own plan in May -- conservative and moderate GOP wings balking at different parts -- but with a much smaller margin of error to pass his version of the plan, H.R. 1628. With 50 votes, a tie-breaker by Vice President Mike Pence would ensure passage.

Nevada Senator Dean Heller said Friday he’ll oppose the bill in its current form largely because of its cuts to Medicaid and to subsidies for individual insurance coverage. Heller, the Senate Republican seen as most at risk in the 2018 midterm election, also said he’s likely to vote with Democrats to block it from floor consideration.

On Sunday’s television talk shows, two other moderates -- Susan Collins of Maine and Bill Cassidy of Louisiana -- said they can’t say yet how they’ll vote and expressed reservations about McConnell’s speedy deadline for action.

Collins said she’s "very concerned" about the bill’s impact on Medicaid coverage for the sickest people. She also opposes its one-year ban on funding for Planned Parenthood, a provision she wants to strike on the Senate floor.

"It’s hard for me to see the bill passing this week, but that’s up to the majority leader," Collins said on ABC’s "This Week."

Conversely, four conservatives -- Rand Paul of Kentucky, Ted Cruz of Texas, Mike Lee of Utah and Ron Johnson of Wisconsin -- say the plan keeps too much of Obamacare. They announced they’ll need a number of changes before they’ll back it, and Johnson said Sunday the rushed process could cause him to join Heller in voting to block the start of debate.

"I have a hard time believing Wisconsin constituents, or even myself, will have enough time to properly evaluate this for me to vote for the motion to proceed," he said on NBC’s "Meet the Press."

The Congressional Budget Office will issue an analysis as early as Monday spelling out how many Americans may lose health insurance under the bill and whether it contains enough financial wiggle room to let McConnell try to woo holdout senators with increased funds.

Related: AHCA continues to bring out the critics

The CBO said the House version, which includes $834 billion in Medicaid cuts and $664 billion in tax cuts, would cause 23 million Americans to lose their health insurance by 2026. A reduction in the number of people losing insurance under the Senate bill might bolster support for it.

‘Vote-a-rama’

After talks with lawmakers, McConnell will probably introduce a revised bill in the Senate Tuesday evening, said Senator John Cornyn of Texas, the No. 2 Republican leader. That would set off a 20-hour debate later in the week, followed by an all-night “vote-a-rama” to consider potentially hundreds of amendments -- including many designed by Democrats to be political booby traps -- before a final vote early Friday morning.

Related: Senate GOP health bill to break with House on key points

While some senators have been "more public about their concerns" with the measure, "they want to get to yes," Cornyn told reporters Sunday at a Koch Brothers donor gathering in Colorado Springs, Colorado.

Senator Richard Burr of North Carolina, the Intelligence Committee chairman and a McConnell ally, went from doubting whether the Senate could get a bill done this year to predicting passage.

"A few weeks ago I said it was difficult for me to see how it gets done. And I came back, and they pulled the Band-Aid off and now it’s going to get done," Burr said.

The showdown may get delayed until after next week’s July 4 recess, though, because some senators -- including Johnson -- are seeking more time to examine the measure.

Cruz’s demands

Cruz was ready Thursday with a list of changes he’s demanding, which included letting states design coverage without needing federal waivers, allowing consumers to buy insurance across state lines, and allowing insurers that meet federal mandates to sell other plans that don’t comply.

Other senators also want revisions. Rob Portman of Ohio said the bill’s $2 billion to combat opioid addiction isn’t nearly enough. Alaska’s Lisa Murkowski joined Collins in expressing concern about the one-year ban on funds for Planned Parenthood.

The new bill made relatively modest changes to the measure that passed the House last month.

Senate leaders added funds in the first few years aimed at bringing down insurance premiums and shoring up Obamacare marketplaces. An extra $62 billion over eight years would go to a state innovation fund, which can be used for coverage for high-risk patients, reinsurance and other items.

Austerity push

To appeal to conservatives, the Senate plan would impose longer-term austerity on health spending. In 2025, it would go further than the House in capping federal Medicaid funds by using the consumer price index, rather than a measure of medical inflation that tends to rise more quickly.

Related: 5 things for agents to know about the Senate health bill

Over time, the extra help for states and the insurance exchanges would phase out, and cuts to Medicaid and to insurance protections would phase in.

While the bill provides far more assistance to the poor and near-poor than the House bill, it’s still substantially less generous overall than the Affordable Care Act. The biggest losers would be people at 351 percent of the federal poverty level -- a little over $42,000 for an individual -- who would go from paying a maximum 9.7 percent of their income in premiums to having no cap at all.

The Senate also keeps almost all of the House’s tax cuts intact -- allowing Democrats to argue that the measure finances tax cuts for the wealthy with reductions in health care for other people.

McConnell has flexed significant partisan muscle even before starting work on the health plan. In April, he ignited the “nuclear option” to confirm President Donald Trump’s Supreme Court nominee, Neil Gorsuch, changing Senate rules permanently to bar filibusters of high-court picks. Last year, McConnell refused to consider former President Barack Obama’s nominee, Merrick Garland, gambling successfully that a Republican would win the presidency and get to fill the seat.

The majority leader is taking heat from Democrats and many in his own party for secretly drafting the health bill with a small group of congressional aides. It isn’t clear how much that hurts his chances of replacing Obamacare, but there is some risk others will join Heller and vote with Democrats to block consideration.

"We don’t have enough information -- I don’t have the feedback from constituencies who will not have had enough time to review the Senate bill," Johnson said Sunday. "We should not be voting on this next week."

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