WASHINGTON — He has played the role of Hillary Rodham Clinton, Al Gore and Barack Obama. He has spent hours studying intricate details of their policy positions, perfecting the cadences of their voices and refining the lines of attack against their Republican opponents.

For more than a decade, Senator Rob Portman, Republican of Ohio, has deployed an unusual expertise: impersonating Democrats to help candidates in his own party prepare for debates. He has frustrated George W. Bush, agitated Dick Cheney and pushed John McCain beyond the brink of irritation.

“I hate him still,” Mr. McCain said with an admiring air of sarcasm, laughing in a brief interview as he recalled their testy practice sessions four years ago. “Because he’s such a jerk, he gets under your skin, says things that are impossible to respond to.”

But this year, Mitt Romney may have another part in mind for Mr. Portman.

The search for Mr. Romney’s running mate is entering its final phase, and Mr. Portman, who has ascended to senator from staff member during nearly three decades in Washington, is believed to be high on the list of prospects. He has campaigned repeatedly with Mr. Romney, and he is flying to New Hampshire this week — where the Romney family is vacationing — to appear at a Republican fund-raiser.