I suspect that almost any Obama pick would draw significant Republican opposition–which is as it should be, since the Democrats are committed to a view of the judicial role that ought to be opposed. The further left the nominee is, though, the higher the no-vote count will be: Obama could lose the votes of Sotomayor supporters Alexander and Bond, for example. A further-left nominee could also stir up more public opposition.
All that said, if I were a liberal president, I’d be tempted to go for it.