The first is that Mr. Obama's other big challenge is convincing moderate Americans he shares their values. He is already seen by many as a liberal, big-city politician who says people cling to guns and religion out of bitterness, associates with radicals, and attended a church with a radical theology. Mr. Biden is a fierce foe of gun rights, ardently opposes restrictions on abortion that have widespread support and promotes gay rights. He supports higher taxes, bigger government and socialized healthcare. That doesn't exactly help Mr. Obama with blue-collar voters in Ohio, Pennsylvania and Michigan.

The second is Mr. Biden's lack of executive experience. Not only has he never been a governor or a cabinet secretary, he has never been a mayor, an agency head, or served in any other executive role, not even prosecutor or military officer. Given that Mr. Obama also lacks that experience, having two career legislators heading the executive branch of our government might create doubts.

And the third issue is Hillary Clinton. Many of Senator Clinton's supporters are still unhappy with Mr. Obama, and right now he is losing some of them to John McCain. This pick does nothing to help heal that rift, which could make the difference in a close election.

More broadly, it cuts against Mr. Obama's central campaign theme of change. His message is Washington is broken, and the old establishment needs to be swept away in favor of new blood and a new vision. How does picking someone who has been in Washington a decade longer than Mr. McCain jive with Mr. Obama's contention that Mr. McCain has been in Washington too long to change it?

Mr. Biden was not the best choice for Mr. Obama. Sure, Messrs. Bayh and Kaine certainly had problems of their own. But if he wanted someone with broad-spectrum national security/foreign policy credentials who could reach traditional voters, he could have asked Sam Nunn of Georgia. If he wanted executive experience and appeal to new constituencies while still getting foreign policy expertise, he could have asked Bill Richardson. And if he wanted to heal his party, he could have asked Hillary Clinton.

Mr. Obama has made his choice, and it's interesting. Now all eyes are on John McCain.