Thomas Donohue, president of the U.S. Chamber of Commerce (Joshua Roberts - BLOOMBERG)

Congressional candidates trot out endorsements  all the time. Most of them mean nothing. But every once in a while, we come across one that grabs our attention.

That happened today in California's 52nd district, where the U.S. Chamber of Commerce backed Rep. Scott Peters (D) against Republican challenger Carl DeMaio.

The endorsement is notable for two reasons.

One is that the Chamber, which is the nation's largest business organization, simply doesn't back Democrats with the frequency it did in the not-too-distant past. Peters is just the fourth Democratic congressional candidate the organization has decided to support this year, according to Chamber spokeswoman Blair Latoff Holmes. The other three are Reps. John Barrow (Ga.), Henry Cuellar (Tex.) and Jim Costa (Calif.).

In 2010 2008 the Chamber backed 38 Democrats. In 2010, it supported 21. In 2012, that number slipped to five.

One of the reasons for the shrinking pool of Chamber-backed Democrats is there aren't nearly as many moderate, business-friendly Democrats in Congress as there were in 2008. Another is the Chamber's apparently intensified focus on helping Republicans win.

The second reason the Peters endorsement is notable is because he is one of the most vulnerable Democrats running for reelection. Judging by the Cook Political Report's race ratings, he's in more trouble than Costa, Cuellar and Barrow.

If you're wondering why the Chamber backed Peters, here's one reason: He cast friendly votes 69 percent of the time in 2013, according to the group's scorecard -- aligning him more closely with the organization's interests than most Democrats.