WASHINGTON — With Senator Obama the likely Democratic nominee and Senator McCain long his party’s presumptive nominee, the search for their vice presidential picks can now begin.

Below, the five most logical vice presidents, assuming Mr. McCain and Mr. Obama are the candidates, ranked in the order of their likelihood of being chosen.

REPUBLICANS:

5. Mitt Romney: A few months ago, it would have seemed crazy to include Mr. Romney on a list of potential vice presidential picks. It was an open secret that he and Mr. McCain didn’t like each other, but Mr. Romney is charting an aggressive fund-raising schedule on behalf of Mr. McCain during the coming months.

4. Governor Crist of Florida: Mr. Crist’s endorsement of the senator from Arizona just before the Florida primary put Mr. McCain over the top and cemented his grip on the nomination.

3. Rob Portman: Mr. Portman, who spent 12 years in Congress, hails from Ohio — perhaps the swingingest of swing states this fall — and is widely acknowledged as an expert on economics.

2. Senator Thune of South Dakota: Mr. Thune comes across as a moderate, despite his very clear conservative voting record. Mr. Thune is also a hero in conservative circles, thanks to his defeat of Senator Daschle in 2004. The one knock on Mr. Thune is that he hails from a state that is already well in hand for Republicans.

1. Governor Pawlenty of Minnesota: Mr. Pawlenty has been elected twice in a Democratic-leaning state that is almost certain to be a battleground. He is liked and respected by both conservatives and moderates. He has also known Mr. McCain for nearly three decades and, at 47, could allay some concerns about Mr. McCain’s age.

DEMOCRATS:

5. Sam Nunn: Mr. Nunn is one of the pre-eminent Democratic thinkers on foreign policy and defense issues. He spent more than two decades in the Senate representing Georgia and chaired the Armed Services Committee.

4. Governor Kaine of Virginia: A former missionary and a man eager to talk about his faith, Mr. Kaine could help Mr. Obama bridge the “God gap” that has emerged in recent presidential elections. He is also the highest-ranking elected official in Virginia, an emerging battleground state.

3. Senator Clinton of New York: After Mrs. Clinton’s speech in Indianapolis last Tuesday, many within the party thought she was opening the door to the idea of sharing the ticket. Her rhetoric since then, however, particularly her comments about “white voters,” may well quash the “Dream Ticket” talk. Picking her as vice president would also seem to run counter to Mr. Obama’s change message.

2. Governor Strickland of Ohio: Mr. Obama is well aware of the need to offer an olive branch of sorts to the backers of Mrs. Clinton. The first-term governor of Ohio, Mr. Strickland is an active and high-profile supporter of Mrs. Clinton. He is also the popular chief executive of a state that Mr. Obama must win.

1. Governor Sebelius of Kansas: Picking the second-term Kansas governor would affirm Mr. Obama’s core message of change and would give his run even more historic weight. Ms. Sebelius’s electoral success in traditionally Republican Kansas would also echo Mr. Obama’s pledge to change the electoral map in the fall.