The answer is that you can blame BOTH Obama and the Senate. You don't have to let Obama off the hook just because the Senators are owned outright by their campaign sponsors. And Obama has done horrible things all by his lonesome that the Senate came nowhere near. HAMP, anyone? Want another example? The Senate didn't force him to call for assassinations of American citizens without due process, did it? This idea that Obama would be the perfect progressive president if it weren't for that meddling senate is just silly It's not an either/or proposition.
The Senate didn't force Obama to appoint like Larry Summers and Tim Geithner, who got the stimulus wrong. It didn't force him to escalate in Pakistan and Afghanistan. Didn't force him to continue and amplify the Bush Wall Street bailouts -- without asking for anything in return. Didn't force him to negotiate the public option away with big hospital lobbyists. Didn't force him to open up offshore drilling. Didn't force him to sign a weak FinReg bill that papered over Too Big to Fail. He called the bill awesome!
It also didn't prevent him from adopting a confrontational stance with the banksters and it did not force him to negotiate with Mitch McConnell exclusively on the tax cuts for millionaires deal. And it didn't force him to pull bullshit, right-wing frame affirming stunts like freezing federal wages.
Of course Lieberputz and Ben Nelson are problems. But I never hear Obama blame them. When he complains about anyone, he complains about DFHs.