FRANK MANKIEWICZ, who knows a little about intense constituencies from his battlefield experience as director of Sen. George McGovern's 1972 campaign, had this to say after President Nixon's landslide reelection victory: "We were always subject to this pressure from the cause people; we reacted to every threat from women or militants or college groups. If I had it to do over again, I'd learn when to tell them to go to hell."

Former California governor Ronald Reagan, as far as we can determine, does not seek counsel from Mr. Mankiewicz. But the lesson Mr. Mankiewicz learned in 1972 might have particular relevance for Mr. Reagan in 1980.

As evidence of Mr. Reagan's long lead in the race for the Republican presidential nomination, there have already been printed a number of stories about his selection of a running mate. God knows we have no list of candidates of our own, nor does Mr. Reagan (who has done rather well so far without it) really need our advice.

But the off-to-the-right weekly Human Events, which has had a far clubbier relationship with Mr. Reagan, has already examined one of the men on the vice-prsidential "list" and found him thoroughly unacceptable. You may not have been aware (we were not) that Mr. Reagan's choice of Senate Minority Leader Howard Baker "would be perceived by many conservatives as a betrayel." Sen. Baker, in spite of a recent 60 percent favorable rating from the American Conservative Union, "is repeatedly," according to Human Events, "found on the liberal side of issues." Tell it to the proponents of SALT II.

Why is this? Because there is one real litmus test that Sen. Baker failed and for which he cannot be forgiven, according to the political theologians of Human Events: "While Reagan vigorously fought the giveaway of the Panama Canal in 1976, Sen. Baker, as everyone knows, played the key role in getting the Senate to approve the giveaway."

Perhaps it is the all-too-familiar quality of this entire discussion that made us remember the words of Frank Mankiewicz. It will be interesting to watch whether Ronald Reagan will, if and when he has the chance, "react to every threat" or whether he will "learn when to tell them to go to hell."