Major Garrett reports that it was pressure from three high-powered House Democrats that finally forced Hillary Clinton into setting a date to suspend her campaign for the nomination.

Congressional sources tell Major that Reps. Charlie Rangel of New York, Barney Frank of Massachusetts, and Norm Dicks of Washington leaned on Hillary Clinton to end her campaign this week, after she declined to give a date for her departure on a conference call with about 20 congressional supporters

Rangel, Frank and Dicks made separate appeals to Clinton to come up with a plan to leave the race and throw her support behind Obama, telling her the party needs to unify.

All three were dissatisfied with Clinton’s speech on Tuesday, when Clinton failed even to acknowledge Obama’s insurmountable delegate lead. On Wednesday, Rangel told ABC News that “Unless she has some good reasons– which I can’t think of– I really think we ought to get on with endorsements and dealing with what we have to deal with… so we can move forward.”

Clinton later told congressional backers that she’d move to begin unifying the party soon, but the of lack a specific date caused Rangel, Franks, and Dicks to ratchet up the pressure

Overnight, Clinton sent a letter to supporters letting them know about her Saturday event where she’ll thank those who have been behind her for the last 16 months and express her support for Obama’s candidacy. You can read the full letter after the jump.

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