Why? In part, I think, because the chances of congressional Democrats reclaiming the U.S. House aren’t especially good. Therefore, if control of the Senate changes hands -- and control of the House does not -- President Obama’s lame-duck presidency will essentially commence overnight. From RCP:
Recall that when the president was running for re-election he smashed fundraising records and spent tons of time on the campaign trail. Former White House Press Secretary Robert Gibbs, then, would like to know what happened to that sense of urgency that was on full display two years ago. Obama’s not involving himself in the political process or “getting voters excited” as he once did, Gibbs argued; however, the president has thrown some 45 political fundraisers since winning re-election. Thus, it cannot really be said he's sitting idly on the sidelines, either.
Still, the Democrats risk losing the Senate in part because of unfavorable terrain. Several prominent Senate Democrats are up for re-election in red states. Unfortunately for them, too, they have to contend with and defend their votes in favor of Obamacare -- a law that is consistently and stubbornly unpopular. It could be that the White House has made a difficult but necessary calculation: the president is a persona non grata on the campaign trail now. By their own admission, some Democrats have already conceded this point in public.
In a move cheered by Republicans nationwide, former Massachusetts U.S. Sen. Scott Brown on Friday declared he wants to "stop complaining and get involved again" by formally joining the race against Democratic U.S. Sen. Jeanne Shaheen.
The longtime Massachusetts resident, who recently moved into his seacoast New Hampshire vacation home, launched an exploratory committee to enter the Senate race during a Republican conference in Nashua, ending months of speculation about his intentions.
Of course, Republicans need to (net) win six Senate seats and keep the House to de facto end the Obama presidency -- no easy task. But with the electoral map shifting under Democrats’ feet -- and the president seemingly disengaged -- perhaps Robert Gibbs' fears of a Republican takeover this fall are not entirely misplaced.