If he wins reelection, Obama may finally be able to dictate the terms of a bipartisan debt-reduction deal. And if he loses to Republican Mitt Romney, Obama could make sure that tax rates rise before he hands over the keys to the White House on Inauguration Day in late January.

Administration officials declined to say whether the veto threat will stand if Obama loses the election.

Obama has never explicitly said whether he is prepared to let the new year arrive without taking action to avoid the cliff. Some Republicans, noting that the president has backed off demands for higher taxes twice in the past, are skeptical that he will stand firm now. But his veto threat challenges Republicans to a dangerous game of chicken over a fiscal event that would raise taxes for nearly 90 percent of households, slice deeply into military and domestic budgets, and probably spark a brief recession.