From Ezra Klein here:

This is all about stopping a law that increases taxes on rich people and reduces subsidies to private insurers in Medicare in order to help low-income Americans buy health insurance. That’s it. That’s why the Republican Party might shut down the government and default on the debt.

And also this:

Democrats won more votes than Republicans in the last election. That was true in the presidential campaign. It was true in the Senate campaigns. And it was true in the House campaigns.

That doesn’t mean the Republican Party is under any obligation to stand back and let Democrats do as they please. But imagine if the Republican Party had won the 2012 election and Senate Democrats threatened to breach the debt ceiling and cause a financial crisis unless Republicans added a public option to Obamacare. Does anyone think a President Mitt Romney would find that position reasonable? Does anyone think that position would be reasonable?