Charles: The key passage of that letter is this one:
In the event that Republicans make good on their threat by failing to act, or by moving unilaterally to pass a debt limit extension only as part of an unbalanced or unreasonable legislation, we believe you must be willing to take any lawful steps to ensure that America does not break its promises and trigger a global economic crisis — without congressional approval, if necessary.
So: It’s okay if Republicans pass a debt-ceiling increase, or pass a debt-ceiling increase with “balanced” spending cuts and tax increases, but it’s “hostage-taking” (they use the metaphor) to pass it with spending cuts and no tax increases. That’s the alleged principle here. “Taking a hostage” just means attaching policy conditions they dislike, in other words.
Update: David Weigel of Slate comments. I don’t see how anything he says is responsive or even pertinent, but maybe I’m missing something.