Four months into his tenure as Majority Whip, Rep. Roy Blunt (R-Mo.) is losing his longtime top aide.

Gregg Hartley, who previously worked for Blunt in Missouri and has served as the lawmaker’s chief of staff since 1997, has informed his boss that he will soon be leaving the Hill.

Hartley declined to comment for this story, but sources said he plans to leave Blunt’s office within the next two months and does not yet have his next job lined up, though he will likely have several suitors. The Missouri native recently turned 50 and is said to be eager to spend more time with his family.

Those who know Hartley well praised his hard work and unwavering loyalty to Blunt.

“The most important thing that Gregg brought to the job is good chemistry with Roy Blunt,” said Dan Mattoon, a lobbyist at Podesta/Mattoon who previously worked as deputy chairman of the National Republican Congressional Committee. “It’s important for the chief of staff to be an alter ego to the boss, and that’s what Gregg achieved.”

Blunt has not yet begun a full-fledged search for Hartley’s replacement, and it is not clear whether he will promote someone from within, hire an aide from another Republican office, look to K Street or try to woo a staffer from the Bush administration.

Blunt currently has two deputy chiefs of staff. One is Mildred Webber, who previously worked for then-Majority Whip Tom DeLay (R-Texas) and the NRCC while also running several leadership races. The other is Brian Gaston, whose resumé includes stints with then-Majority Leader Dick Armey (R-Texas) and the House Republican Conference.

Aides who have worked closely with Blunt’s operation said Hartley would be leaving the office in fine shape.

“One of the things that Gregg leaves behind is that he’s helped Roy put together a top-notch staff,” said Susan Hirschmann, ex-chief of staff in DeLay’s Whip office and now a lobbyist at Williams & Jensen. “They have a top-notch political staff and a fantastic Whip team.”

Assuming he chooses to go to the private sector, Hartley is likely to be highly sought-after on K Street. In addition to his long service with Blunt, Hartley is close to a broad spectrum of Republican Members and aides. He also has strong ties to the Bush administration, as Blunt is one of the White House’s top allies on the Hill.

Wherever he ends up, Hartley is expected to remain an informal adviser to Blunt and will likely retain his role in the Whip’s political operation.

“During his off-duty time Gregg has been a tremendous fundraiser,” Mattoon said.

Hartley first worked for Blunt from 1989 to 1993 when he was Missouri secretary of state. Before that, he served in the office of then-Missouri Gov. John Ashcroft (R).

After a stint in the private sector, Hartley followed Blunt from Missouri to become his House chief of staff in 1997. He remained in charge as his boss came up through the ranks from backbencher to Chief Deputy Majority Whip to his current post.