John McCain has the Republican nomination sewn up, and while Barack Obama and Hillary Clinton continue in their struggle to lead the Democrats, the Arizona senator can turn his attention to other matters.

Interactive: Short List | Survey

One of the most important: Who will be his vice presidential running mate?

McCain might use his pick to add youth to the ticket. Although he has joked about his age on the campaign trail, he would be 72 years old on Inauguration Day, older than any other president-elect in history.

And with an independent streak that rankles the Republican base -- columnist Thomas Sowell, for example, calls him a "loose cannon" -- his choice could bring ideological balance to the ticket.

The choice of a vice presidential candidate must also be seen as an act of wisdom. Who would be able to step in and lead the country should the president die or resign?

Youth? Ideology? Experience? Use this Interactive to see who might be on McCain's short list.