Barack Obama's world tour is over, and by most accounts it was pretty successful (Gallup's daily tracking poll shows a nice bounce). But the McCain campaign has found a way to rain on Obama's parade, with a new ad, "Troops," that slams the Democratic nominee for canceling a visit to wounded American soldiers in Germany. Obama had planned to visit the troops at Landstuhl Regional Medical Center, but after the Pentagon voiced concern about the appropriateness of bringing a campaign staffer with him, Obama decided the event had become too politicized. In the ad, a narrator says that Obama "made time to go to the gym, but canceled a visit with wounded troops. Seems the Pentagon wouldn't allow him to bring cameras. John McCain is always there for our troops." The ad has been criticized on a number of fronts: While Obama does seem to spend a bizarre amount of time in the gym, he didn't actually go to the gym instead of visiting the troops in Landstuhl. Plus, the ad actually shows a photo of Obama playing basketball with American troops … which would seem to undercut the idea that Obama is too busy to meet with the troops. To some, the ad simply smacks of desperation.

• Chuck Todd and friends note that, like the ad blaming high gas prices on Obama, "Troops" is "a bit over the top; it asks the voter to assume something unlikely — that Obama doesn’t care about U.S. troops." How do attacks like these affect McCain's "straight-talk image?" [First Read/MSNBC]

• Ron Claiborne says the ad, whose origins can be seen in comments made by Sean Hannity, contains "an extraordinarily harsh attack line." [ABC News]

• Vaughn Ververs says the ad "is the latest example of an increasingly aggressive (dare we say 'negative') tone" in response to Obama's recent success. But McCain could "risk losing one of the few advantages he has," which is his general likability and his ability "to float above his party label for the most part." [ Horserace/CBS News]

• Ben Smith and Jonathan Martin write that the ad is part of a new McCain strategy to hit "Obama not on his record or his platform, but on his values and person." [Politico]

• Ari Melber thinks the attack makes McCain look "small" for "seizing on a meaningless non-event, exploiting American troops by casting them as victims in this petty squabble." The controversy is another "complete diversion from the actual foreign policy issues at stake." [HuffPo]

• Joe Klein believes this type of ad is what you fall back on when you're "desperate" and you "apparently have nothing substantive to say about America's future role in the region and the world." [Swampland/Time]

• Kevin Drum contends that McCain's "desperation for the presidency [is] driving him to conduct a campaign that's carefully but relentlessly testing ever more contemptible depths of squalor in its attacks on Barack Obama." It's sad to watch a man who cares so much about his dignity abandon it for a campaign. [Political Animal/Washington Monthly]

• Juliet Eilperin writes that at least one Republican with ties to the McCain campaign says his team is "not happy with where they are and they're lashing out," and that the line of attack "hardly will resonate with the swing voters who are going to decide this election." [WP]

• Art Levine thinks the Obama campaign should have seen attacks like this coming and done a better job at preventing and responding to them. [HuffPo] —Dan Amira

For a complete and regularly updated guide to presidential candidates Barack Obama and John McCain — from First Love to Most Embarrassing Gaffe — read the 2008 Electopedia.