WASHINGTON (AP) - Rick Santorum is endorsing his one-time bitter rival Mitt Romney in a late-night e-mail to his supporters.

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Santorum on Monday urged his supporters to join him in working with the effective Republican nominee to deny President Barack Obama a second term. In his message, Santorum reminded his supporters of areas where he disagreed with Romney during their slog of a primary.

Yet Santorum says that, in his words, "above all else, we both agree that President Obama must be defeated."

During their contest, Santorum bitterly clashed with Romney. At one point, Santorum called Romney the worst Republican in the country to challenge Obama.

The two met on Friday at the office of Santorum's strategist, and Romney sought to assure Santorum of his conservative credentials.