WASHINGTON -

President Barack Obama's decision to name retiring Democratic Sen. Max Baucus as ambassador to China sets off a chain reaction that could give the White House and Democrats an edge in preventing Republicans from gaining a Senate majority.
    
The beneficiaries could be Montana's Lt. Gov. John Walsh, a Democrat seeking Baucus' seat, and, thanks to the dominoes of Senate seniority, Sen. Mary Landrieu of Louisiana, who is facing a difficult re-election.
    
The advantages would be on the margins, potentially giving the two Democrats a slight boost. But in a series of close elections with Republicans needing to gain a net of six seats to recapture control of the Senate, any lift for Democrats could help them retain control and protect Obama's agenda.