Originally Published: 16 JUN 21 13:59 ET

Updated: 16 JUN 21 14:41 ET

By Manu Raju and Clare Foran, CNN

(CNN) -- Sen. Joe Manchin, a key Democratic swing vote, would not commit on Wednesday to backing an infrastructure package along straight party lines, rebuffing a demand made by liberals who want to ensure he will be on board with a broader bill passed without Republican support if they support his bipartisan proposal.

"It should not be conditional," Manchin told CNN. "It should be based on the merit of what the bill is."

The West Virginia Democrat did not rule out supporting a bill passed with only Democratic votes, but argued that asking him to commit to backing legislation advanced through reconciliation -- a procedural tool that would allow Democrats to pass a bill with a simple majority -- now is not fair.

"I don't think that's fair," he said.

"I'm not a no, not a yes," Manchin said of legislation advanced strictly along party lines. "I'm evaluating everything."

The comments from the centrist Democrat threaten to upend a careful balancing act by Democratic leaders as they implement a dual track approach to President Joe Biden's signature priorities on infrastructure.

Democrats are encouraging efforts to strike a bipartisan deal that would appeal to moderate members of their party, like Manchin, while at the same time laying the groundwork for a bill passed solely along party lines that could incorporate elements of Biden's infrastructure agenda left out of any potential bipartisan agreement.

Many Democrats, including progressive and liberal members, have signaled that they would not be willing to support a bipartisan deal unless they have a firm commitment that a separate bill will also be advanced along party lines via reconciliation. But every single Senate Democrat would need to be willing to vote for legislation passed through reconciliation for it to succeed given the 50-50 partisan split in the Senate, which leaves Democrats with zero margin for error.

When pressed on the demand from liberals, Manchin suggested he would not ask the same of them if the tables were turned.

"I would never ask any of my colleagues for ironclad commitment," Manchin said, "unless I could justify it and show them the facts of why I would hope they would work together with me, agree with me, I would, and I expect the same from them."

As a result, if Manchin is unwilling to commit to supporting legislation passed through reconciliation, progressives may feel they do not have the assurance they need to support any bipartisan effort, which could, in turn, derail passage.

Democrats have extremely slim majorities in both chambers, including the narrowest possible majority in the Senate, which leaves party leaders in a position where they cannot afford to lose many, if any, votes from both moderates as well as progressives.

Manchin pushed back on the idea that there would be some sort of a "trade off" to secure votes on both a bipartisan bill and a Democrat-only bill.

"I've never been in that position on any bill. I never try to trade off," he said. "Unless you know it's something that we all agreed on."

Manchin said of the bipartisan effort, "We think we put a good deal together. We're working on it. We're gonna meet again today on making sure that we have everybody in agreement on this and then move forward and then if they have a better solution to it that we can move forward, we should."

This story has been updated with additional developments Wednesday.