Let's put aside the question of why Judd Gregg would bail on his fellow Republicans in the Senate, since he's reportedly said he won't accept Obama's offer of the Commerce Secretary post unless John Lynch, New Hampshire's Democratic governor, pledges to appoint a Republican to replace Gregg in the Senate. Instead, let's turn to the question of why Lynch would agree to such a deal.
It seems as if there's no real downside to his appointing a Democrat to Gregg's seat--and getting his party 60 votes in the Senate. Lord knows New Hampshire Democratic Congressman Paul Hodes, who was already gearing up to challenge Gregg in 2010, would take the job. So why doesn't Lynch just appoint him? Lynch is already popular--he trounced his Republican opponent by nearly 50 points in 2006--so it's not as if he needs to worry too much about appeasing New Hampshire Republicans. Could it be that Lynch himself is interested in Gregg's seat? He's denied any interest suddenly desire Gregg's seat, wouldn't the smart play* be to appoint a caretaker Republican as Gregg's replacement--somebody like Bonnie Newman-- and then run in an open field in 2010? Just asking.
*--Yes, Lynch could just appoint himself as Gregg's replacement, but that seems a bit hamfisted, especially so soon on the heels of his denial that he's at all interested in going to Washington. If, say, 10 months from now he announces he's changed his mind and he now feels it's his calling to run for the U.S. Senate, people won't even remember his original denial.