(CNN) -- Pennsylvania Rep. Joe Sestak has informed supporters he plans to challenge Sen. Arlen Specter in the Democratic primary, a spokesman for the congressman tells CNN.
Jonathon Dworkin, Sestak's communications director, confirmed the authenticity of a handwritten letter from Sestak - first posted on the Web site Talking Points Memo - to a supporter in which the congressman states, "I intend to run for the U.S. Senate."
Dworkin also confirms quotes from Sestak's sister to TPM stating, "He intends to get in the race,..."In the not too distant future, he will sit down with his wife and daughter to make the final decision."
Sestak’s decision to challenge Specter, who recently switched his party affiliation from Republican to Democrat, puts the congressman at odds with Democratic congressional leaders and President Obama. The president and the congressional leaders have endorsed Specter for re-election in 2010.
Sestak will appear on CNN's The Situation Room to discuss the race at 6:45 p.m. EST.
Text of the letter:
"I am writing you as especially dear supporters to let you know I intend to run for the U.S. Senate...my candidacy's credibility will have much to do with my fundraising success by the 30 June FEC filing deadline at the end of this quarter. Would you help me bring the change for the future we Pennsylvanians need[?]"