The future of a fiscal cliff deal is now up to Senate leaders as they try to forge a tax and budget agreement, sell it to each party, avoid a series of ever-present procedural hurdles, get it approved by the Senate and then get that measure to the House for final action by New Year's Day.

"Discussions are underway," was how it was characterized to me on Saturday morning, as I was cautioned not to expect any announcement on what might be in this plan before the still fluid details are run by Senators on Sunday.

The behind the scenes maneuvering on Capitol Hill quickly brought questions from my listeners about the constitutional legitimacy of such a plan, as people rightly point out that revenue bills are supposed to start in the House.

"How can the Senate initiate legislation on taxes first?" read one email on Saturday morning.

It's pretty simple, really.

The Senate just has to take a House-passed tax or spending bill, strike out the text of that measure and insert the language of whatever is agreed to by Senate leaders.

That just happened over the past two weeks in the Senate, as they took a defense spending bill from last year, and replaced the text with the Hurricane Sandy disaster aid plan.

That revised bill was approved by the Senate on Friday evening and will now go back to the House for further action.

As of this writing, I don't know which House-passed bill the Senate will use as the legislative "shell" or "vehicle" for a fiscal cliff deal, but there are a number of bills available.

The most obvious choice might be a bill that the House passed back on August 1, H.R. 8, which would extend the Bush tax rates for all Americans for one more year; the vote was 256-171.

That bill was sent to the Senate, where it was placed on the legislative calendar. It has not been brought up for any amendments or votes, but could easily become the legislative vehicle for a fiscal cliff deal.

The Senate is not in session on Saturday; both the House and Senate will return to work on Sunday.

But expect a lot of behind the scenes work by staffers of both Senate Majority Leader Harry Reid and Senate Republican Leader Mitch McConnell, as they try to put together a tax and budget agreement and then present it to Senators on Sunday.

The fiscal cliff deadline of January 1 is not far away.