The fate of a social spending and tax bill agreed to by Sen. Joe Manchin and Senate Majority Leader Chuck Schumer hangs in the balance as another key moderate, Sen. Kyrsten Sinema still hasn't said if she'll support it.

Manchin, D-W.Va., said he plans to talk with Sinema, D-Ariz., on the Senate Monday about the bill. But her office says it may take a little longer for the senator to decide how she will vote. 

"Sen. Sinema does not have comment as she’s reviewing the bill text and will need to see what comes out of the parliamentarian process," a spokesperson for the senator told Fox News Digital Monday.

With a 50-50 Senate, and Republicans coalescing against the Manchin-Schumer bill, every vote will count for Schumer, D-N.Y., and his bare bones majority to pass it.

Kyrsten Sinema

Sen. Kyrsten Sinema hasn't announced where she stands on a social spending and taxation bill agreed to by Sen. Joe Manchin and Majority Leader Chuck Schumer. (Reuters/Caitlin O'Hara)

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That means Washington is waiting with bated breath to see where she comes down on what may be the Democrats' last and best chance to pass a social spending bill before the midterms. Extending the drama, Sinema's announcement may not come until later in the week, due to the nature of the parliamentarian process her spokesperson referred to.

Senate Parliamentarian Elizabeth MacDonough this week will hear arguments from Republicans and Democrats over whether certain parts of the bill comport with the Byrd Rule. That process is called the Byrd Bath.

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That rule says, generally, provisions in a reconciliation bill – which is the process Democrats are using to get around the 60-vote filibuster – should affect government spending or revenue.

MacDonough will rule on which provisions don't fit that definition, making them what the rule calls "extraneous matter." Technically, the Senate could overrule the parliamentarian, but top lawmakers haven't given any indication they plan to do so. 

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With a bill hundreds of pages long for the parties to bicker over, it's possible that the process won't be wrapped up until later in the week. 

That means it could be several days before Sinema announces her final position, which could either dash Democrats' hopes of a major legislative victory or power them to what could be their biggest win yet.