Tea Party extremists seek to fabricate a false fiscal crisis making millions of Americans economic hostages, in order to demand cuts to Social Security and elderly nutrition assistance programs, defense cuts, medicare cuts, and cuts to many other programs.
Spending is already authorized by Congress. Failure to raise the debt ceiling forces the President to decide which law to violate, or to resort to extreme measures such as minting trillions in money. Ultimately, default on US debt would cause a worldwide recession and endanger national security.
The US Constitution says that US debt obligations shall not be questioned. Threatening to default on the US debt in order to exact leverage that could not be gained in fair elections violates the Oath congressmen swear to uphold.