Rep. Dennis Kucinich (D-Ohio) will officially exit the presidential race today and endorse his one-time rival and presumptive Democratic nominee John Kerry, sources close to the Massachusetts Democrat’s campaign confirmed Wednesday.

Kucinich, the liberal standard-bearer who entered the White House bid on a platform largely centered on the war in Iraq, is the final Democratic challenger to bow out of the race. He had vowed early on to remain in the running until the Boston convention next week, even after it was assured he had no chance of capturing his party’s nod. Kucinich still controls commitments from 68 delegates.

Kerry now heads to Boston as the uncontested Democratic presidential hopeful, having already won the support of all his other rivals for the nomination.

“Dennis is going to be a tremendous asset in Ohio and a number of other battlegrounds,” said one senior Kerry strategist, who confirmed Kucinich would jump on board today.

Kucinich said earlier this week he planned to back Kerry and running mate John Edwards, and believes the greatest strength of the party is its unity. Even though Kucinich represents the party’s progressive wing, he said he believes it is best to work within the party to push his agenda and ideals.

“Each of us made a commitment to support the nominee,” Kucinich said of the presidential hopefuls. “That’s a commitment we made even when [Howard] Dean looked like he was going to be the nominee.”

The Ohio lawmaker added that his focus is on unifying the party, and backing Kerry achieves that goal. And Kucinich also said — even before formally backing the ticket — he would campaign on behalf of his former opponent and try to bring progressive voters on board with him.

“My purpose is to unite the party and in that unity elect John Kerry and John Edwards,” Kucinich said.

Kucinich was just added to the lineup of convention speakers, landing a Wednesday night prime-time slot.