THE EMERGENCE OF Barack Obama as the Democratic presidential nominee has changed the most important criterion in selecting a vice presidential running mate, notably for John McCain. This has given two Republicans in particular a serious shot at being picked: deputy House Republican whip Eric Cantor and former Bush budget director Rob Portman.

The criterion in this case: the readiness to step into the presidency should anything happen to McCain (assuming he wins the election). The vice presidential candidate must not only be a plausible president but someone who would be recognized as such by the political community and the media.

How has Obama changed this criterion? He's made it easier to meet. If Obama, having served less than four years in national office and then only as junior senator, is a credible president, so are a number of Republicans whose experience or stature is limited. Obama, of course, earned his presidential credentials the hard way--by winning the Democratic nomination.

In the Democratic presidential primaries, Obama's lack of experience as a national leader was used against him relentlessly by Hillary Clinton. She assumed experience was an Obama vulnerability. Her attacks didn't work, even when she cited his failure to hold a single hearing as chairman of a Senate foreign affairs subcommittee. The result: Experience has been downgraded as a factor in the 2008 presidential race.

Cantor, 45, and Portman, 52, have more experience than Obama. Cantor, a conservative from Richmond in his fourth term, has risen rapidly in the House Republican leadership and is extremely

Portman served six terms in the House, from 1993 to 2005, before President Bush named him special trade representative and then budget chief. Portman is a seasoned national figure whose experience dwarfs Obama's. So both Cantor and Portman meet the plausible president test in 2008.

Three other criteria matter. Would the vice presidential running mate hurt the ticket in any way? Would he or she help it, possibly by bringing victory in a battleground state? And would the nominee, McCain, be comfortable with the person on the ticket?

Cantor's attractiveness as a running mate has increased now that Virginia has emerged as a tossup state. He would give McCain a major boost, probably enough to win Virginia handily. And Cantor would also help in pivotal states with significant Jewish populations, Florida and Pennsylvania especially.

Portman has the important political asset of being from Ohio. He moved back to the state--his home is outside Cincinnati in southwest Ohio--when he resigned as budget director last year. Ohio is a must-win state for a Republican presidential candidate, just as Pennsylvania is for a Democratic candidate.

That brings us to another possible running mate for McCain, former Pennsylvania governor Tom Ridge. He was considered a plausible president even before Obama made that criterion less stringent. Like Cantor and Portman, Ridge is a Republican with whom McCain should be quite comfortable. But he has one drawback: He's pro-choice on abortion.

That means on the second criterion, hurting the ticket in some way, Ridge would be a liability. But there's a way to around this: the Bush precedent. That's George Bush senior, who was Ronald Reagan's running mate in 1980. Reagan's choice of Bush threatened to drive away millions of pro-life voters. Bush had been gradually moving toward a pro-life stance, but he hadn't quite gotten there. And he did oppose Reagan's support for a constitutional amendment banning abortion.