It would be a risky move, but assuming a somewhat reasonable nominee in the context of who is being replaced the Republicans could approve as quickly as possible, hopefully win the election and then when the next vacancy opens up they make sure to get "Scalia's replacement" in at that point. Outside of the rulings that might be lost between now and then this would be a net positive for Republicans. Clearly it would be silly for Republicans not to say they intend to block as it's a negotiation at this point. If Obama waits to long it becomes reasonable to block, but putting the ball in his court allowing him either to make a mistake by not making a reasonable nomination or make a mistake by giving the Republicans to much is the move to make. Obama's best move might be to bet on a Democrat win and then nominating his ideal choice a few weeks after the election, although he would have to make sure the Democrat nominee is on board with the plan.