If you’re hardy, stay up until 1am ET tonight and watch the US Senate take a vote on a debt limit plan. The plan will retain most of the structure of what Harry Reid has put forward in recent days – a full $2.4 trillion increase in the debt limit, along with near-term spending cuts in the form of a cap on discretionary spending for around $1 trillion, war savings and assorted mandatory spending cuts for another $1.2 trillion, and the enactment of a joint committee on deficit reduction (what we’ve been calling Catfood Commission II) armed with the authority to recommend future deficit solutions, which will get an up or down vote in both houses of Congress without amendment or filibuster.

The Reid bill didn’t pass last night because it didn’t have 60 votes – he could not persuade enough Republicans to join him on the initial plan. So Reid has been adding sweeteners to get Republicans on board. There are three major possible pieces to sweeten the pot:

1) Reid would adopt Mitch McConnell’s fallback plan to force the President to take all the responsibility for increasing the debt limit. How it would work is that the President would submit the request for the increase, which either House could vote against with a resolution of disapproval. The President could veto that resolution, and it would be sustained with just 1/3 of either chamber.

2) Reid would add another sprinkling of spending cuts, designed to get to the $2.4 trillion number in total deficit reduction. It would do that based on a January baseline, not the March 2011 baseline with the changes from 2011 appropriations. Compared to that (the actual number), the savings would be $2.2 trillion.

Now, if it stopped there, you would basically have McConnell-Reid. There would be spending cuts, but about half of them on the side of war funding accounting. There would be a Catfood Commission, but the debt limit wouldn’t be tied to it. There would be the McConnell disapproval process, but that’s really just a political action designed to force bad votes, and it doesn’t really even force bad ones (Democrats could let as much as 20 votes go in the Senate on such a vote, meaning nobody in trouble for re-election would need to support it).

But if this were totally acceptable, McConnell-Reid would have passed a week ago. So the clear endagme here is a trigger, something that would force the Catfood Commission recommendations into law, with real consequences if it failed. In this negotiation, lawmakers are having the same problem they had with a grand bargain. Republicans don’t want taxes in the trigger, and Democrats don’t want it to be all automatic cuts. We’re having the Catfood Commission debate inside the trigger for the Catfood Commission.

Most think the likely compromise is a trigger that would impose automatic, across-the-board cuts in spending if the committee fails in its mission. But Senate Democratic leadership isn’t so sure. They worry that a spending-cuts only trigger is heads, Republicans win; tails, Democrats lose. “The idea of triggers with just cuts is a non-starter. Republicans would just deadlock the committee and get the cuts they want,” continues the aide. “If there is a trigger it would have to be balanced.”

The White House, according to Ezra’s sources, is less dogmatic on the need for a balanced trigger. But the logic of the Senate leadership aide is impeccable. If the trigger puts into motion automatic and deep spending cuts if the Catfood Commission fails, there would be lots of incentive on the Republican side to let that commission fail. Even if the automatic cuts are untargeted, even if they hit Social Security and Medicare and Medicaid, I can’t see Republicans caring much about that, as long as their precious taxes are spared.

In fact, it may never get to a Congressional vote. The Republicans on the Catfood Commission are unlikely to allow for any taxes, in all likelihood. That could deadlock the commission. And it would weigh on Democrats on the commission who would be in the position to go along with some all-cuts recommendation that is more intelligently targeted, rather than random automatic cuts. An all-cuts trigger just makes for a variety of unpalatable options.

Meanwhile, all of this is predicated on the idea that the Senate passes the first stage of something at 1am Sunday and the House goes along with it. The House today is going to vote down the old version of the Reid plan, with almost all if not all Republicans opposed. They had to make their bill more right-wing to pass it with Republicans. The obvious solution is to allow some Reid-McConnell hybrid that passes the Senate go up for a vote, getting a majority of Democrats and a sprinkling of Republicans to get to 216, but there’s no guarantee at all that John Boehner would allow then.

So the options then appear to be: 1) a Reid-McConnell hybrid with no trigger on the Catfood Commission (unlikely), 2) a bad Reid-McConnell hybrid with a trigger (more likely), 3) default on payment obligations (perhaps the most likely at this point). And the best case scenario, number 1, would be a real reduction in GDP and increase in unemployment as a result of near-term austerity, which could only get worse after the Catfood Commission makes its recommendations.

See you at 1am!