Mike Allen at the Politico is reporting that Connecticut Sen. Joe Lieberman, who appeared on the 2000 Democratic presidential ticket alongside former Vice President Al Gore, will endorse Arizona Sen. John McCain tomorrow.

Lieberman, who lost the Democratic primary for his seat, was elected to another term as an Independent, and usually caucuses with Democrats in the Beltway. McCain and Lieberman, according to Allen, will appear together on the Today show on NBC tomorrow morning at 8 a.m.

The endorsement does little harm to either man. Those who voted to re-send Lieberman to the U.S. Senate did so with an understanding that he was not going to tow either party line. Those who still have McCain on their short list may not see the endorsement as a big win, but it should pull the more moderate, pro-war factions of the parties to give the Arizona Senator a second look.

With very limited research, however, it does appear this endorsement is groundbreaking in that this marks the first time a candidate on one party’s highest ticket has crossed the aisle to endorse an individual from the opposite party, also on the highest ticket.