The fight between Republicans and Democrats over reopening the government and raising the debt limit is not even really about policy anymore.

It's all about teaching the other side a lesson.

Democrats think (correctly) that Republicans have settled on making terroristic threats over the economy as a strategy to gain policy concessions that cannot ordinarily be obtained just by controlling one half of Congress.

They are determined to punish Republicans so they won't employ this strategy in the future. Part of that punishment comes in the form of Republican poll numbers tanking, which has already happened. And part comes from ensuring that Republicans get absolutely no policy wins to show for their hostage-taking.

An outcome is only acceptable to Democrats if it clearly demonstrates that the hostage-taking strategy failed and Republicans were idiots for trying.

The Senate has not passed a bipartisan compromise bill yet, but Senate Democrats have been clear that any concessions in the bill will be reciprocal: Democrats will only give something small up (like new income verification requirements in Obamacare) in exchange for a small give from Republicans (like delaying a fee on insurance plans that unions hate), not in exchange for raising the debt ceiling itself.

When Senate Democrats talk to Republicans, they're Michael Corleone:

Or Willy Wonka:

Republicans feel disrespected by a president and a Senate Majority Leader who won't negotiate with them over Obamacare in the context of a debt ceiling fight. They believe (however delusionally) that they have the backing of the American people and that Democrats aren't being fair. And they hate the president and his health care law and feel the need to take both down a peg.

Coming away from this fight with nothing will be hugely wounding to their egos and enraging to their base. They have to come away with something, to show the President that they matter and their concerns will be addressed.

There is plenty of room for compromise on substantive policy issues. But there is no room for compromise on the question of, "Can Republicans get something by threatening the country with default?" The answer to that question can only be yes or no; one side has to win and the other side has to lose.

There is a reason behind this fight beyond questions of ego and economic stability. It goes to who has what power in a divided government.

A divided government has to pass spending bills at least once a year with support from both parties, and that leaves considerable room for each party to make demands about discretionary spending. That's why, over the last two years, Republicans have more or less gotten their way on discretionary spending, which declined this year and will likely decline next year, too.

But for mandatory spending programs like Social Security and Obamacare, which don't have to be reauthorized every year, divided government produces a major status-quo bias. If the two parties can't agree about what to do with these programs, they stay in whatever way the law already says they should. And Democrats are broadly satisfied with the legal status quo in these programs, while Republicans want big changes, including repeal of Obamacare.

For that reason, Republicans have a reason to try to find a way to break that dynamic, and they've settled on using the discretionary appropriations process and the debt limit — matters that controlling one house of Congress does give you leverage over — to demand changes in mandatory spending.

In other words, their strategy for getting entitlement reform is saying, "Hand it over or the economy gets it." This situation enrages Democrats partly because it's terrible for the country. But it also enrages them partly because it endangers a structural advantage that they get as defenders of the mandatory spending status quo.