WASHINGTON — It is the morning of Wednesday, Nov. 7, and after a long night of celebrating and a short night of sleep, Mitt Romney and Paul Ryan wake up to confront the question awaiting every new administration: Of the campaign’s many promises, which few will become the real priorities?

If they win the White House, Republicans are also more likely than not to hold on to the House of Representatives and win a narrow majority in the Senate. The party could then embark on the kind of aggressive legislative push that President Obama and the Democrats did in 2009.

Only four years after Democrats seemed on the verge of historic policy gains, Republicans could reverse many of those gains and then some. They could cut the top tax rate to its lowest level in 80 years (as Mr. Romney proposes) and make major changes to federal programs.

Above all, a sweep of Washington could make possible the sort of change Republicans have been talking about for three decades without having yet done: a significant shrinking of government. Ronald Reagan cut domestic programs somewhat but expanded the military, while George W. Bush talked about small government but actually made it bigger. Next year, though, really could be different.