Rep. Jim Courter of New Jersey has succeeded where a lot of other Republicans--including First Daughter Maureen Reagan--have failed. He got a letter of endorsement last week from President Reagan for his primary.

Some 70,000 copies of the Reagan letter are going to prospective voters in the June 8 New Jersey congressional primary, where Courter jumped in against Morris County Freeholder Rodney P. Frelinghuysen. The letter is unstinting in its praise of the second-term congressman, hailing his "caliber of service, leadership and judgment."

How did Courter, who had hoped Gov. Thomas Kean would appoint him to former senator Harrison Williams' seat, become the only Republican to gain the primary election endorsement of a president who has made a policy of neutrality in intraparty contests as long as he has held public office? Why would Reagan do for Jim Courter what he would not do for his own daughter in her June 8 California Senate primary?

White House aides say that the key was the unanimous recommendation by House GOP leader Bob Michel, Congressional Campaign Committee Chairman Guy Vander Jagt, National Chairman Dick Richards and New Jersey GOP Chairman Philip D. Kaltenbacher.

Another factor, they suggest, was Courter's willingness to move into new territory, rather than run against fellow Republican Reps. Matthew J. Rinaldo or Marge Roukema, who took over his old territory in the redistricting.

Everyone was happy with Courter's decision except Frelinghuysen, who says he won't hold the endorsement against Reagan, if he comes to Washington.

His father, ex-Rep. Peter Frelinghuysen, reportedly is less charitable, however--livid about his son's being the victim of Reagan's only breach of neutrality.