Centrist Democratic Sens. Joe Manchin (W.Va.) and Heidi Heitkamp (N.D.) are not ruling out voting for the Senate GOP tax plan, even though the chances of them doing so appear slim.

Manchin and Heitkamp, who are both up for reelection next year, have not yet decided whether they will vote against the motion to proceed or against the bill on final passage. Both told reporters they can't say for sure until they know all the details of the legislation.

"We haven't seen the final version. We think they're still trying to find ways to get 51 votes," Manchin said.

Republican senators say they don't expect Manchin or Heitkamp to get them to the 51 votes, but they think they might support the measure in the end if the bill appears headed to President Trump's desk.

Heitkamp said "it's unfair to ask that question" when asked whether she would oppose the tax bill on final passage.

"I've been asking all along what is it and I still don't know what it is. It's still a moving target," she said.

Heitkamp said she was concerned about a number of provisions in the legislation, including what she called the front-loading of tax relief to the nation's wealthiest taxpayers

"We've got some challenges ahead and some questions on how we do this," she added.

When asked whether they could even vote to begin the floor debate and amendment process on the tax bill, the centrists declined to say.

Other centrist Democrats have their minds firmly made up about the GOP plan, which would cut the corporate tax rate to 20 percent, double the standard deduction and add an estimated $1.5 trillion to the deficit over a decade, among other things.

"I would vote twice against this bill," said Sen. Michael Bennet (D-Colo.).

Manchin, Heitkamp and other Democrats oppose the proposal to lower the corporate tax rate to 20 percent, a central component of the GOP bill, but they say they are willing to negotiate.

Manchin, who has proposed setting the corporate rate at 25 percent, organized a press conference of 16 Democrats and one independent, Sen. Angus King (Maine), calling on Republicans to work across the aisle.

They pledged they would find a way to pass permanent tax reform that could pass the upper chamber with 60 or even 70 votes.

"The group of us are saying to our colleagues and our friends on the Republican side, 'Please, we want to work with you,' " Manchin said.

"We all want to do tax reform, we all believe it needs to be done," he added.

Many of the Democrats who attended the event have a record of working with Republicans on difficult policy problems, such as Sen. Tim Kaine (Va.) on the authorization of military force, Sen. Joe Donnelly (Ind.) on the medical device tax, and Sen. Maggie Hassan (N.H.) on opioid addiction.

Donnelly, another Democrat up for reelection next year, said the current tax bill falls short of what he, Manchin and Heitkamp discussed with Trump at a White House dinner in September.

"We told the administration our focus is keeping jobs here in this country, ending outsourcing, standing up for the middle class and making sure this does not explode the debt," Donnelly said.

"That's not what this legislation is," he said of the GOP bill scheduled for a preliminary vote in the Senate Budget Committee Tuesday afternoon.