Paul Clement used to argue for the federal government's power until he started arguing against it.

But he's no flip-flopping political candidate; he's a lawyer. Changes like this are part of his job.

Clement is playing a key role in three politically charged Supreme Court cases in which Republican-led states object to Obama administration policies or federal laws on health care, immigration and redrawing political boundaries.

In the biggest of those, the 45-year-old law school acquaintance of President Barack Obama will be trying to sink Obama's health care overhaul.

Not that long ago, Clement would regularly stand before the justices and defend even the most aggressive uses of federal power, making his case without written notes and parrying questions with an easy banter.

He argued for the Bush administration's policy on detaining suspected terrorists, a federal law outlawing a medical procedure called "partial-birth abortion" by opponents, the McCain-Feingold law aimed at limiting the influence of money in politics and a federal ban on the use of marijuana for medical purposes.