If Mr. Romney wins, Republicans say they would seek to delay the year-end deadline for a bipartisan deal by up to a year to give him time to flesh out his budget plans and get Democrats to agree. But even if Democrats and the financial markets go along with the delay, the months before Mr. Romney's swearing-in could be as crucial to his presidency as the transition period was for Mr. Obama four years ago, when the economic crisis led him to draft a big stimulus package while President George W. Bush still occupied the White House.

Mr. Romney's ability to foster cooperation at the outset could determine his success on a range of issues. Yet Democrats have been dismissive, at best, about his budget plans, which have few specifics on how Mr. Romney would reduce deficits. He has mostly spoken about cutting taxes and increasing military spending.