(The Supreme Court just this week agreed to referee such a battle. The case of M.B.Z. v. Clinton tests a statute ordering the State Department to record that American citizens born in Jerusalem are born in "Israel," though official U.S. policy doesn't recognize Jerusalem as part of Israel. Both Bush and Obama administrations have refused to issue the birth certificate as the statute requires, on the grounds that the Constitution gives the executive branch the sole authority to recognize foreign governments and determine their jurisdiction. The lower court held that the dispute was a "political question" -- meaning one that courts could never resolve. The Court will reconsider that holding; if it finds the question "justiciable" -- meaning a court can answer it -- it may still hold that the executive was right to disobey the statute.)
To me, one interesting thing about my imagined scenario -- Obama defying the debt limit and ordering Geithner to borrow if needed to repay America's debts -- is how closely it actually fits recent practice by Republican presidents. For the past three decades, conservative theorists have insisted that the executive branch is the rightful place for every federal power except those few given exclusively to Congress. That was the Bush administration's constant refrain during the early years of the War on Terror. In one of the infamous "torture" memos during the weeks after September 11, Office of Legal Counsel official John Yoo breezily wrote that "the constitutional structure requires that any ambiguities in the allocation of a power that is executive in nature -- such as the power to conduct military hostilities -- must be resolved in favor of the executive branch." That meant, to Yoo, that the president could attack any nation he chose, on his sole determination that it posed or might pose a threat. Congress could authorize him to attack another country, but could not forbid him to do so.
Surely the power to safeguard the national credit is "executive in nature," too. It is commanded by the Constitution, and it concerns the national interest as fully as does military action. Congress can authorize the borrowing of funds to repay the debt; but can it prevent the president from doing so by refusing to act?
I'm not sure I buy the executive-hawk argument; but sauce for the Bush goose, an administration lawyer might argue, should be sauce for the Obama gander. If the Bush White House could, on its own authority, judge that the Detainee Treatment Act was unconstitutional when applied to limit the president's authority to employ torture, why shouldn't the Obama White House make the judgment that the statutory debt limit is unconstitutional if it requires "question[ing]" the national debt?
This is how the imperial presidency came to be, and why it keeps growing. As historian Daniel Walker Howe notes in his marvelous book What Hath God Wrought: The Transformation of America 1815-1848, divided government became a persistent feature of American politics during the administration of John Quincy Adams; it was Adams's successor, Andrew Jackson, who tackled the problem by asserting sweeping authority for the president to act regardless of Congress when he in his sole judgment saw fit. In the American system, coalition government and shared responsibility are difficult; executive power grabs are quick, clean -- and usually successful.