Washington: Finding a running mate tops the summer to-do lists of John McCain and Barack Obama, and each needs to decide what he wants as much as who he wants for vice president.

Are they out to offset their own perceived weaknesses, or reinforce an area of strength? Are they looking for someone from a particular region or demographic niche? How much should personal chemistry count in the equation? It's all about achieving balance.

The end of the Democratic primary season this past week touched off considerable debate about the relative merits of an Obama-Clinton "dream ticket". Obama could decide to pick vanquished rival Hillary Clinton on the strength of her campaigning skills, or steer clear of the political baggage she and her husband would bring. He could view adding her to the ticket as a way to reach out to women and unify the party, or see her as a divisive figure who might turn away general-election voters.

A similar balancing act will play out on the Republican side. McCain, 71, could try to use his running mate to bulk up the ticket's economic credentials, for example, or to inject some youthful vigour.

There are names afloat for just about any scenario, and Clinton's is surely one of the most buoyant.

"She is hardworking; she is tough; she is very smart," Obama said recently. "So I think she'd be on anybody's short list of the vice presidential candidates, but beyond that, I don't want to offer an opinion."

If Obama wants to amplify the ticket's sense of history by adding a woman, Clinton is not the only choice. Also on the list of potential candidates are Governor Janet Napolitano of Arizona and Kathleen Sebelius of Kansas and Missouri Senator Claire McCaskill.

For McCain, there are fewer Republican governors and fewer Republican lawmakers in Congress to include in the mix. Reagan administration hand A.B. Culvahouse is advising McCain on the process, according to Republicans close to the campaign.

McCain spent the Memorial Day holiday weekend in Arizona with four potential contenders: Charlie Crist, governor of Florida, a crucial state in November; one-time rival Mitt Romney, the former Massachusetts governor; Louisiana Governor Bobby Jindal and Kansas Senator Sam Brownback.