“Sometimes party loyalty asks too much,” JFK once said. Obama supporter Cory Booker is learning that lesson the hard way.

The Democratic mayor of Newark famously wandered off the reservation by calling President Obama’s attacks on Bain Capital and private equity “nauseating.” For his trouble, he ended up in the Obama woodshed and emerged to recant in a hostage-style video.

But the damage was done — first to Obama, and then to Booker himself. Lacking the courage to break with his party’s dogma, he went on MSNBC, which is like a forced re-education camp, and attacked Romney for using his words in an ad.

The episode went from news to farce to tragedy in a day. Booker, who relies heavily on Wall Street donations and has friends in New York high society, could have helped the Obama campaign by steering it to a higher, smarter level. In fact, he quickly got support from other Dems, including former Tennessee Congressman Harold Ford Jr. and former Pennsylvania governor Ed Rendell.

But under attack from left-wing bloggers and Obama campaign operatives, Booker crawled back to the mothership and ended up looking weak and unprincipled.

His is a textbook example of how politics is like gang warfare without guns. The total mindset of “us-against-them” eliminates anything that smacks of constructive criticism or adhering to a single standard. As in any gang, the lowest common denominator prevails because nobody has the guts to break ranks.

For a shining moment, Cory Booker did. A pity he couldn’t take the heat. He might have made a difference.

To continue reading Mr. Goodwin's column on other topics, including the 9th Circuit Court's convention in Hawaii, click here.