Julian Zelizer: Campaign Rhetoric May Tie Next President's Hands

Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" (Times Books) and of the new book "Governing America" (Princeton University Press).

Princeton, New Jersey (CNN) -- If Barack Obama and Mitt Romney are staying up late worrying about whether they can win the election, they should ponder another, ultimately more important, question: Will their campaign rhetoric make it impossible for them to be effective if elected president?

The decisions that each man makes in his effort to defeat the other will shape the political environment in January 2013. Although we often consider the campaign phase of a presidency to be entirely separate from governing, the truth is that the two are intimately connected.

Whoever takes office in January will face many difficult challenges that will force him to compromise, adjust and move away from campaign promises that no longer fit the reality of the times. The Bush tax cuts expire at the end of the year, along with the payroll tax cut designed to boost the economy. The pressure will be on for the president and Congress to make deep spending cuts and revenue increases.

The president's health care law will either need to be implemented and funded, or it will have been ruled unconstitutional, thus pushing to the forefront once again the skyrocketing costs of health care. In foreign policy, the Middle East, Iran, North Korea and China all point to hot spots that are volatile and unpredictable. And these are just the known challenges, let alone the crises we can't yet see coming.

For Obama, the dangers are significant....