If he were a king, then I would agree. But, he's not (he has an annoying thing called "Congress" to deal with). With a Brown win, Obama would lose his filibuster-proof majority in the Senate and I'll bet dimes to dollars that he is going to see huge losses ten months from now in the House and Senate elections. With such losses, he would be forced to work with Congress in a bi-partisan manner (just like Clinton before him).
Neither Bush nor Reagan ever had a filibuster-proof majority in the Senate and look at how each of them still managed to do a massive amount of damage. Nixon managed to do a lot of damage despite having a Democratic congress djuring his entire presidency. The President has vast executive powers and can do a vast amount even if the opposition control both houses of congress. Getting 60 seats in the Senate was a bit of a fluke last year when the Democrats really ran the tables in the Senate - it was never going to last more than two years. As for bi-partisanship - it takes two to tango. We already saw that on health care, the Republican were totally intransigent a refused to cooperate at all. They put zero proposals on the table and basically just said NO to everything. The Senate Finance committee spent months meeting with conservative Democrats bending over backwards to get a couple of Republicans to meet them half way and even after they stripped health care reform of almost everything that the Republicans ever said they opposed - they still had every single Republican vote NO. You can't be "bipartisan" when the other side essentially views you as illegitimate and refuses to talk to you.