The Senate is planning a critical vote Tuesday on Republicans' long-promised health-care overhaul, but on the eve of that decision, Sen. Pat Toomey (R., Pa.) was still unsure what exactly he would be voting on or whether the measure could pass.

The cloud of uncertainty hovered even as Toomey spoke to business owners and executives at a suburban office park in West Chester. And with the potential for failure looming, the senator, who helped write the Republican health plan, already had one eye on another key GOP priority, tax reform.

He told reporters at the roundtable event that he was unsure if Republicans had the votes to keep their current repeal effort alive. Still, Republican leaders vowed to push ahead with a key procedural vote, with success looking like a long shot. Should they advance the measure anyway, it's unclear what would come next — whether they would move to repeal and replace the law known as Obamacare, to simply repeal the law and replace it later. The uncertainty has left several senators wary on a vote on which the GOP has almost no margin for error. Either way, party leaders hope to force senators to formally take a stand before summer recess.

Toomey, like other Republicans, did not know which idea would be posted for a vote.

"I expect to find that out, actually quite soon, sometime this afternoon," he said Monday after a meeting set up by the Chester County Chamber of Business and Industry. Acknowledging the uncertainty, he later added, "It's a big, complicated challenge."

Adding to the challenge: Several major provisions critical for winning over conservative votes could be dropped under the complex rules that Republicans are using to try to pass the health measure with a simple majority, rather than a 60-vote threshold.

Toomey said that he would prefer to repeal and replace the legislation at the same time, but that he was open to the idea of repealing now and then trying to work with Democrats on a replacement. The GOP would delay the repeal for two years to try to craft a new law.

"If we've got the votes, the ideal thing from my point of view would be to have as much of the replacement as we can do done now," he said. "But if we don't have the votes, if we could in the alternative pass a repeal bill where the repeal goes into effect at some date in the future and we stabilize the market in the meantime, I do think there's a chance that the Democrats would begin to be willing to work with us in that scenario."

With the GOP on the brink of failure to deliver on a seven-year old promise, President Trump turned up the pressure Monday afternoon at the White House, where he appeared with people who he said had been hurt by the law.

"Any senator who votes against starting the debate tells America you are fine with the Obamacare nightmare," Trump said, delivering the robust public push that his critics say has largely been missing during the halting repeal effort. "There's been enough talk and no action.  Now is the time for action."

Democrats, citing projections by the nonpartisan Congressional Budget Office, have warned that the GOP health plan would add more than 20 million to the ranks of the uninsured and could undermine consumer protections for people with expensive pre-existing conditions.

Republicans faced steep odds. They can afford to lose only two of their 52 votes and advance their plan — but got a boost late Monday night when Sen. John McCain (R., Ariz.) announced he would return Tuesday after receiving cancer treatment, giving the GOP one more vote. Republicans, however, still face skepticism from several other senators who have attacked the bill from the center and the right.

Toomey suggested he might lobby wavering Republicans. "There's a few colleagues that I think are genuinely undecided, and I might be able to play a constructive role," he said.

He added that the repeal push would not end if it stalled Tuesday.

"I don't think it's over because it's too important," Toomey said. "… So we can't just ignore it and walk away from it."

If the vote fails Tuesday, it will likely mean going back to the drawing board and a process that could turn into a slog as Republicans focus elsewhere.

"I think the tax code is holding us back, and there is a fairly strong interest on the part of Republican senators, the administration, and the House to pass fairly comprehensive tax reform to deal with the business aspect of our tax code as well as individuals," Toomey said. "So whether or not we're successful on health care, we're going to move on to tax reform very, very soon."