The opposition from the left is a moral one based on the sketchy shit about him. He's running for senate. He can serve on committees and write bills. That's the extent of it. What are folks worried he'd actually do?

Opposition from the right is that he'll vote for the progressive things he says he will.

Left is concerned about who he is (purity testing), the right is concerned about what he'd do (because it'd be bad for them).