Mayor Bloomberg is poised to endorse Washington Mayor Adrian Fenty’s re-election bid at a D.C. restaurant tomorrow, the Washington Post reports.

The relationship between the Democratic D.C. mayor and the Democrat-turned-Republican-turned-independent mayor of Gotham dates back several years.

In 2009, Fenty and a fellow up-and-coming African-American elected official, Newark Mayor Cory Booker, endorsed Bloomberg’s successful bid for a third term – a fact about which the Bloomberg campaign made much, thinking it would help eat in to the base of support for Bloomberg’s Democratic opponent, then-NYC Comptroller Bill Thompson.

Mo Elleithee, a senior strategist for Fenty’s rival in the mayoral race, Vincent C. Gray, responded to Bloomberg’s impending endorsement thusly:

We are happy to concede the Manhattan vote to him. We’re going to keep focusing on the residents across all of DC’s 8 wards.

Bloomberg apparently is going to do a round of out-of-state endorsements of Democrats tomorrow, as he’s also scheduled to back – and campaign with – Joe Sestak, who is fighting to keep a Pennsylvania US Senate seat in Democratic hands.