U.S. Rep. Joe Sestak, trying to knock off a veteran Democratic incumbent senator in the primary, will get an endorsement Monday from somebody who succeeded in doing just that: Connecticut's Ned Lamont.

Lamont defeated Sen. Joseph Lieberman in the 2006 Democratic primary, largely by running against the Iraq war and Lieberman's support of the Bush administration's war policy. But Lamont did not win the general election. After losing the nomination, Lieberman ran as an independent in the fall, defeating Lamont and the Republican nominee.

Sources close to the Sestak campaign confirmed that Lamont is the "mystery endorser" it announced would be coming to Independence Hall Monday.

Sestak, a two-term House member from Delaware County, is seeking to defeat Sen. Arlen Specter, who joined the Democratic Party in April, in the 2010 primary.

Contact staff writer Thomas Fitzgerald at 215-854-2718 or tfitzgerald@phillynews.com.