Senate Republicans hold a critical vote Tuesday on the latest effort to repeal Obamacare, in the face of withering criticism from President Donald Trump—and confusion over whether they even have enough support to move forward.

Trump has spent weeks cajoling, strong-arming and warning Republicans to get on board with his effort to overhaul his successor's health care reforms, but several are skeptical about how the new plan could affect millions of Americans.

In recent weeks, several measures have been considered—but ultimately collapsed, revealing fissures within the Republican Party on how to reach a goal they have had since the Affordable Care Act was passed in 2010.

"Big day for HealthCare. After 7 years of talking, we will soon see whether or not Republicans are willing to step up to the plate!" Trump tweeted early Tuesday before the vote, expected after 2:00 pm (1800 GMT).

The latest Republican plan would dismantle Obamacare but delay actual implementation of the repeal to allow time for a viable replacement to be crafted.

That bill is unlikely to pass in its current form, but the Senate leadership has stressed the importance of at least voting on it in order for changes to then be introduced.

In a sign of how important the numbers are for the effort, Senator John McCain, who was convalescing in Arizona following a brain cancer diagnosis, returned Tuesday to the Senate to cast his vote.

Republicans hold 52 of the Senate's 100 seats. With all Democrats opposed, Trump can afford just two defectors.

In the event of a 50-50 vote, Vice President Mike Pence would break the tie.

As of last week, three Republicans said they would vote no on Tuesday's motion to proceed, which would open debate on the legislation. Leadership heaped pressure on the holdouts.

"We can do better than Obamacare," said Senate Majority Leader Mitch McConnell, who highlighted McCain's return as he urged fellow Republicans to close ranks.

"Today's vote to begin debate is the first step, and we should take it," he said.

On Monday, in a stark warning to fellow Republicans, the president bluntly declared "Obamacare is death" and that the Senate must act.

"Any senator that votes against starting debate is telling America that you are fine with the Obamacare nightmare," he added.

'Hard, calculated rhetoric'

Trump has repeatedly grilled fellow Republicans for not following through on their—and his—campaign pledge to repeal and replace Obamacare.

As some Republicans expressed frustration with the process, and skepticism about what the Senate might pass, others were eager to hold the vote and see where colleagues stood.

"Now is the time to prove that our rhetoric of the past seven years regarding the repeal and replacement of this disastrous bill will lead to substantive action," Senator Ted Cruz said in a statement.

McConnell and other Republican leaders acknowledge they do not know whether there is sufficient support to even open debate on the new plan, a sign of Republican division about the impact such reforms might have on millions of American families.

On the Democratic side, senators urged cooperation—and restraint.

"I can't believe this process and the hard and calculated rhetoric we see," Democratic Senator Joe Manchin of West Virginia said Tuesday.

"The Affordable Care Act is not a perfect piece of legislation. It needs repair. We both agree to that. Then we need to fix it."

Secretive process

Forecasts by the non-partisan Congressional Budget Office on various health reform bills have predicted that millions of Americans would lose health care if the measures become law.

In the case of a bill that repeals Obamacare and provides no replacement, 32 million more people would be uninsured by 2026 as compared to current law, CBO forecast.

Some Republicans have expressed concern with how legislation would impact Medicaid, the health insurance program for the poor and the disabled.

The latest repeal-and-replace bill would roll back an expansion of Medicaid and slash its federal funding.

It would also end the mandate that most individuals have health insurance, and allow states to let insurance companies offer bare-bones plans not allowed under current law.

Democrats have blasted the secretive process, accusing Republican leaders of rushing a mammoth bill to the floor without sufficient discussion or debate.

Several outside health groups have criticized the various iterations of the repeal-and-replace effort.

One of the most emphatic rejections came from a group of some 7,000 Catholic nuns, who wrote Senate leaders Monday to say the bill "would be the most harmful legislation for American families in our lifetimes."