Gov. Chris Christie’s announcement on Friday that he was endorsing Donald J. Trump startled the political world. But every political marriage involves a bit of calculation, and in this case, each man has much to gain from the alliance.

What Mr. Christie Gains

Relevance:

There is nothing more painful or humbling for a presidential candidate than returning to an old, humdrum job. The phone doesn’t ring. The lights are not so bright.

Now Mr. Christie is back where he wants to be — in the headlines, shaking up a political story, newly enlisted to campaign across the country and soon to be overwhelmed with news media requests. Look for him to become a regular again on MSNBC’s “Morning Joe” and to forget, for now, about New Jersey’s credit rating downgrades.

Revenge:

It is difficult to overstate how angry Mr. Christie remains about the party’s embrace of Senator Marco Rubio of Florida, whom the governor regards as an unqualified lightweight.