No number of Cabinet secretaries from the opposition will save him from the other party's vigorous attacks if they think they can win. But the Democratic party showed him no mercy, even in national-security matters. JFK became a powerful president because his policies were seen to work, and because he handled himself so well. As for bipartisanship on policy matters, there are innumerable matters on which most Americans agree, and the next president should be able to claim the consensus for himself. That's smart leadership, not catering to the consequences of a close win.
Winners govern, and losers look for a job.
There isn't much more to it than that.