President Reagan's long vacation is over, and he is about to embark on what White House spokesman Marlin Fitzwater calls "the busiest month of his presidency."

Some might say Fitzwater is damning with faint praise. But Reagan will certainly be busy enough by any standard during the weeks ahead as he tries to write a satisfying final chapter to the disappointing story of his second term.

The highlights of that chapter, according to the White House outline, will be a U.S.-Soviet treaty eliminating medium-range nuclear weapons and Senate confirmation of Supreme Court nominee Robert H. Bork. The anticipated subplot includes an attempt to muddle through in Nicaragua while the contras remain in existence as a fighting force.

This twilight chapter has two potential heroes, and both are on the spot. One is the president, fresh from his Santa Barbara ranch and ostensibly eager to do battle with adversaries ranging from the Sandinistas to erstwhile friends on the Republican right, who suspect that Reagan has gone soft on communism in his dotage.

Friends say Reagan has privately expressed frustration and even anger at the intensity of the friendly fire. The president complains that some conservatives refuse to understand that improved spy satellites have made it possible to detect Soviet cheating without intrusive on-site inspection. He also complains that old friends give him no credit for genuinely desiring an arms agreement, preferring to pretend that he is being manipulated by the State Department, his wife or the chimera of history.

Despite his supposed combat- readiness, Reagan resists public battle with longtime supporters. He isn't accustomed to maneuvering on the left of the Republican presidential field, where competition centers on the question of who is most distrustful of the Soviets and the Sandinistas. But White House chief of staff Howard H. Baker Jr. has no standing with this gang. Reagan must demonstrate on his own that his support for arms control and peace transcends the anticommunist catechism.

Baker is also on the spot. He has been anathema to the Reagan hard core since he supported the Panama Canal treaties and then, as Senate majority leader, put the New Right's social agenda on the far back burner. Now, Baker is custodian of what remains of Reagan's domestic agenda.

Last week, reflecting on political realities in a telephone interview from his home in Huntsville, Tenn., Baker quoted a local saying that "a man is known by the enemies he keeps." Baker's enemies are not hard to find. The conservative bible Human Events, once Reagan's favorite reading matter, last week urged the return of Donald T. Regan as chief of staff. Others on the right refer to Baker as "Dr. Sominex," or, horror of horrors, accuse him of favoring a negotiated peace in Nicaragua.

However, former senators are not without honor in their own land. An irony of Baker's situation, of which he is aware, is that those who denounce him most fiercely also depend upon him to secure confirmation of their Ollie North hero replacement, the eminent Judge Bork.

"The most important part of a Senate majority leader's education is over by the third grade, when he has learned to count," says Baker, who mastered addition when he was Reagan's point man in the Senate during the halcyon days of the first term. Baker, who views Bork as a reasonable conservative jurist rather than a radical activist who will remake the court, counts nearly 50 senators in the undecided column. A close associate says these votes, given a reasonable performance by Bork in the confirmation hearings, are going to be won or lost "one or two at a time," the kind of inside politics at which Baker has always excelled.

The achievement of peace and freedom in Nicaragua seems an insoluble puzzle, dependent upon the shaky good will of the Sandinistas and the contras and beyond the power of either Reagan or Baker to determine. But a treaty that maintains the momentum of arms control can be accomplished if Reagan is willing to stand up to his friends. And the confirmation of Bork is also within reach if Baker has the strength and skill to ignore his enemies. Don't bet against a senator who learned to count in Tennessee.

Substitute Reaganism of the Week: Fitzwater, appearing at a briefing with a bottle of champagne to celebrate what he called "the 58th consecutive month of economic growth," said: "Reaganomics gets better with age."