Start from the premise that Republicans will refuse any deal that includes significant new revenue and Democrats will realize that that’s just fine from their perspective, as Republicans can either cut a deal with them in December 2012 or all of the Bush tax cuts can expire. Now take a good, hard look at the trigger.

The Joint Committee is charged with finding $1.5 trillion in savings over 10 years. The trigger would only cut $1.2 trillion over 10 years. The Joint Committee is likely to cut Social Security, Medicaid and a host of programs Democrats aren’t going to want to touch if taxes aren’t part of the deal. The trigger exempts Social Security and Medicaid, and $1 out of every $2 in cuts comes from the Pentagon. The Joint Committee is likely to cut a deal without revenue, and Democrats will have to explain to their base why they permitted, say, Medicare cuts while letting the GOP reject tax increases. The trigger lets Democrats blame Republicans for protecting the wealthy in the 2012 election.

There are, of course, good reasons to fear the spending trigger. In a deal, the cuts can be backloaded to give the economy more time to recover. Under the trigger, the cuts will begin immediately. In a deal, the cuts can be made with care, and many of them can be reforms rather than crude reductions in benefits or spending authority. Under a trigger, you trade the scalpel for a hatchet. In a deal, you might be able to extend the payroll tax cut and unemployment insurance. Outside of a deal, both will expire.

But these reasons only hold if Republicans are willing to cut a deal Democrats actually like. It’s a pretty good bet, however, that Republicans won’t want revenues in the final deal and won’t want the Pentagon to absorb half of the cuts. The extension of stimulus matters, but Democrats are going to have trouble trading $150 billion in tax cuts and unemployment benefits now for $1.5 trillion+ in spending cuts later. So unless Republicans are willing to give on some of their core demands or they’re willing to get creative and throw in a public option or something, it’s pretty easy to see how Democrats could decide the trigger is better politics and, at the least, equivalent policy.