Romney didn't say whether he would overturn it if elected. But by noting "it can be reversed by subsequent presidents," he might have sown doubts in the minds of some young illegal immigrants studying the policy.

Obama looks like the bigger risk-taker. He doesn't have many options.

He is constrained by a complex, interrelated and frail global economy, and by a Republican-run House. Together, they severely limit his ability to influence the struggling U.S. economy, which Obama says needs more investments in education, renewable energy sources and other areas.

Using executive powers and persuasion, however, Obama can expand the rights of gays and lesbians in civil and military life; direct Catholic-affiliated employer insurance plans to cover contraceptives; and protect hundreds of thousands of young illegal immigrants from being deported.