Arizona Republican Sen. John McCain hasn’t had a bad couple of weeks. First, the presumptive White House nominee turns the conventional wisdom on its head and outpoints Sen. Barack Obama (Ill.) on the Democratic hopeful’s trip to Europe. Instead of Obama using his photo opportunities with British Prime Minister Gordon Brown, French President Nicholas Sarkozy and German Chancellor Angela Merkel to convince American voters that he’s ready to run this country, his trip nets him surprisingly little beyond criticism about his self-image.

[IMGCAP(1)]And then, a McCain TV spot dictates the terms of the political discussion, drawing Obama into a messy confrontation and successfully driving home an important point about Obama’s substance and readiness for the White House. Sure, most Beltway insiders have bashed the ad, but, given the success of the conventional wisdom so far, that’s probably reason enough to figure that McCain has hit the right message.

But the next big decision that each candidate has to make — the veep — is likely to benefit Obama, not McCain, at least if the names most widely circulated as on the “short list” of potential running mates are correct.

Each of the three Democrats mentioned most often — Virginia Gov. Tim Kaine, Sen. Joseph Biden (Del.) and Sen. Evan Bayh (Ind.) — has very real assets. Any of them would be a good pick for Obama.

Biden would bring maturity and experience. The Delaware Democrat performed well during the 2008 Democratic presidential primary, and his expertise on foreign policy issues, and particularly his generally thoughtful approach to Iraq, would be an asset.

Does Biden tend to be long-winded? Sure. Would he be an asset in adding electoral votes to the Democratic ticket? No. Would he make a Democratic ticket made up of two Senators? Obviously. But so what? None of those things matter nearly as much as the assets that he’d bring to the ticket and, yes, the country.

Bayh has served two terms as governor and is in his second Senate term. He comes from what has been a “red state” in presidential elections, and his moderate record as governor earned him praise even from Republicans. Cautious and reliable, Bayh is a team player who could be counted on not to say the wrong thing. He was a vocal supporter of Sen. Hillary Rodham Clinton (D-N.Y.) during the primaries and picking him could also help unite the party.

Kaine is the purest play for underscoring Obama’s “change” message, since he would bring a non-Washington dimension that neither Biden nor Bayh would bring. And, of course, he comes from a Southern state that until recently was widely regarded as a Republican bastion and that could end up picking the next president.

Of the three officeholders mentioned for the second spot, however, Kaine has the least experience, which includes service on the Richmond City Council, including two terms as the city’s mayor, a single term as lieutenant governor and his current service as governor. Kaine, in short, doesn’t address any Obama experience deficiency.

The Republicans widely regarded as the most likely to be picked by McCain — former Massachusetts Gov. Mitt Romney and Minnesota Gov. Tim Pawlenty — bring much less to the table than do the three Democrats.

Pawlenty, 47, is a personable two-term governor who barely won re-election two years ago. An early McCain supporter, he is conservative enough to make the GOP base happy. But he likely wouldn’t bring Minnesota over into the Republican electoral vote column, and he certainly wouldn’t change the dynamic of the presidential race.

That leaves Romney, who has received more mention than any other Republican recently as McCain’s likely choice, the Arizona Republican’s obvious contempt for Romney during this year’s GOP primaries notwithstanding.

A former governor who is widely seen as comfortable discussing economic issues and who has survived extensive media scrutiny, Romney passes the political “smell test” on stature and on the ability to step into the nation’s top job if need be.

But like Pawlenty, Romney doesn’t dramatically enhance the appeal of the GOP ticket or change the election map. And some of the reasons given for Romney’s alleged appeal border on the absurd.

A number of observers have suggested that Romney could help McCain in Michigan, a potentially crucial state for the Republicans in November. The logic here, I suppose, is that Romney’s father, George Romney, was governor of the state, and Mitt’s ties there could help deliver the Wolverine State to the McCain column.

The last election that George Romney won in Michigan was in 1966 — 42 years ago. Given that only 12.5 percent of Michigan’s population was age 65 or older in 2006, according to a U.S. Census Bureau estimate, relatively few Michigan voters still alive could have voted for George Romney.

And if that doesn’t convince you that the Romney name isn’t a huge asset now and would bring very much political value to the Republican ticket in the state, consider that George Romney’s wife, Lenore, ran for Senate in Michigan in 1970, while her husband’s term as governor was expiring, and she drew less than 33 percent of the vote against Sen. Phil Hart (D).

It may well be that McCain doesn’t need a dramatic pick that changes the race’s landscape. After all, if Obama fails to close the deal with voters, the election could fall in McCain’s lap. Still, Democrats have to be upbeat that their party’s vice president choice could help cement Obama’s narrow advantage in the race.

Stuart Rothenberg is editor of the Rothenberg Political Report.