He's blamed his exit on "partisanship." From whom, the GOP? Keep in mind that the Democrats have the largest majority in Congress that they've seen in many, many years. They shouldn't have that much trouble passing some of their agenda, even if Republicans were almost uniformly opposed. The problem is the intra-party Democrat conflict, between the left and the center.
Keep in mind what the media would be saying if the parties' roles were reversed, and you hit on the nub of the problem. Say that a Republican moderate bowed out -- in the first year of a far-right president's tenure, having put up with the "leadership" of a "jam-down," right-wing Congress.
We'd be hearing all about the "Republican Taliban," and the like -- how the extremes have come to rule the GOP and are systematically making it impossible for moderates to exert their magical, bipartisan influence.
This year, of course, the Democrats have shown an unbridled passion for huge, budget-busting (and unpopular!!!) legislation. Unlike the President, Bayh has actually succeeded in surviving (and thriving) in a fairly Republican state -- and with Scott Brown's election, he could see the writing on the wall.
Sure, he's getting out because of partisanship . . . from his own side. And he's doing it while he still has a viable reputation as a centrist, without either attracting the abuse that would come from the left if he resisted the Obama agenda in the Senate or jettisoning the reputation as a moderate he's cultivated, if he were to knuckle under to the Obama/Reid/Pelosi crew.
By resigning now and forgoing the campaign, Bayh avoids having to choose between two alternatives that could end up damaging him politically in years (and elections) to come: Either defending the increasingly-unpopular Obama agenda on the record to Indiana voters, or courting the vituperation of Obama true-believers by bucking his president and party.