(Adds detail throughout)
By Andy Sullivan
WASHINGTON, Feb 1 (Reuters) - Democrats would not increase their margin of control in the Senate if New Hampshire Republican Sen. Judd Gregg leaves to serve as commerce secretary, Senate Minority Leader Mitch McConnell said on Sunday.
"Senator Gregg has told me that if he were to take this appointment, it would not alter the makeup of the Senate in terms of the majority and the minority," McConnell, a Kentucky Republican, said on CBS's "Face the Nation."
Gregg has emerged as Democratic President Barack Obama's top candidate for commerce secretary and could be named as early as Monday.
McConnell's comments suggested that a deal had been struck to ensure that New Hampshire's Democratic governor, John Lynch, would not name a Democrat as Gregg's replacement.
Democrats currently control 58 seats in the Senate and are leading in one other undecided race, just short of the 60 seats needed to clear Republican procedural hurdles.
Democrats could pass legislation much more easily if they were to reach that benchmark because they also enjoy a wide margin in the House of Representatives.
"Whoever is appointed to replace him would caucus with Senate Republicans, so I think it would have no impact on the balance of power in the Senate," McConnell said.
Republicans in the Senate believe that Gregg would not accept the post without a guarantee that he would be replaced by a member of his own party, a Republican aide said on the condition of anonymity.
A McConnell spokesman had no further details. Gregg aides were not reachable for comment.
Democrats believe Lynch is expected to name a moderate Republican who would deny them total control of the chamber but would often vote with them.
Senate Democrats are not involved in any deal to replace Gregg, said a spokesman for Illinois Senator Dick Durbin, the Senate's No. 2 Democrat.
Gregg faces re-election in 2010 in a state that has voted increasingly Democratic in recent years.
By naming a Republican to replace Gregg, Lynch could avoid a backlash for giving Democrats a stranglehold in the Senate.
He would also avoid the vicious inter-party brawls that erupted when Democratic governors in New York and Illinois appointed replacement senators from their own party.
Lynch aides said it was premature to talk about any replacement for Gregg and declined further comment.
As a Republican Cabinet member in a Democratic administration, Gregg would help Obama fulfill his promise to work more closely with members of the opposite party. Other Republican Cabinet members include Defense Secretary Robert Gates and Transportation Secretary Ray LaHood.
Gregg could help Obama soften resistance from Republicans opposed to the terms of an $800 billion-plus economic stimulus package the new president is trying to push through Congress.
The top Republican on the powerful Budget Committee, Gregg has developed a reputation as a fiscal conservative over three terms in the Senate. He helped craft the Wall Street bailout last year and was one of a handful of Republicans who voted to release the second half of the $700 billion package.
Obama's first pick for commerce secretary, New Mexico Governor Bill Richardson, withdrew from consideration in January in the face of a corruption investigation.
(Additional reporting by Thomas Ferraro; Editing by David Wiessler)