With his Supreme Court parlay last week, President Reagan wins the George Washington Plunkett memorial trophy for 1986. It was Plunkett, the Tammany boss, who said he would like as his epitaph: ''He seen his opportunities and he took 'em.''

When Chief Justice Warren Burger decided -- as he tells it -- to resign from his judicial post so he could prepare for the 1987 celebration of the bicentennial of the Constitution, Reagan did not waste any time pleading that he reconsider.

He met briefly with Justice William Rehnquist -- the man his advisers had suggested he elevate to Burger's post -- and then with Appeals Court Judge Antonin Scalia, the brain trust's choice for Rehnquist's Supreme Court seat. By the end of each meeting, the job had been offered and accepted.

Thus, in two hours of pleasant conversation, the president had done as much to reshape the future laws of this nation as six years of battling with Congress have accomplished in redirecting fiscal policy.

With Rehnquist, 61, and Scalia, 50, Reagan likely has steered the course of jurisprudence along his chosen conservative channel for the next two decades or more. By elevating two judges of commanding intellect and considerable personal charm, he has denied his political critics any plausible grounds for denying them confirmation.

Presidents have always been eager to put their stamp on the Supreme Court, as University of Virginia political scientist David M. O'Brien demonstrates in ''Storm Center: The Supreme Court in American Politics,'' nicely timed for publication later this month. The appointment and confirmation of Supreme Court justices have always been intensely political matters. They must be, given the crucial role of the high court in our system of government.

O'Brien quotes none other than Rehnquist as arguing in a 1984 speech that there is "no reason in the world why a president should not 'pack' the court,'' if you mean by that appointing ''people to the court who are ir,3p6,220 sympathetic to his political and philosphical principles.''

So far as appointments go, Reagan is a bit behind schedule. Over the long span of American history, Supreme Court vacancies have come up on an average of once every 22 months. There are no guarantees, however. Jimmy Carter, luckless as ever, got none. Reagan has had only two in 65 months, and this last one was at least a bit contrived in its timing.

It has been common knowledge in Washington that Burger, now 78, intended to step down as Chief Justice in order to let Reagan name a younger successor to that vital post. An active and intense Republican partisan in his earlier years, Burger -- who had benefited from the judicial patronage of Dwight D. Eisenhower and Richard M. Nixon -- was certainly not going to frustrate the only other Republican to win the White House in more than 50 years.

My guess was that he would preside over the Constitution's big birthday next year while still wearing judicial robes, and then step down. But he may have been prompted to advance his resignation date by fears that the Democrats will take over control of the Senate this November. If the Supreme Court of Mr. Dooley's day could be said to ''follow the election returns,'' then why should today's high court not monitor the public opinion polls? And those polls increasingly say the Democrats may recapture the Senate.

O'Brien says that of 26 Supreme Court nominees refused confirmation, only two ''suffered defeat because of mediocre judicial records and lack of professional qualifications.'' The other 24 ''have fallen prey to partisan politics, either because of the nominee's political views or because the Senate wanted to deny 'lame duck' Presidents appointments to the Supreme Court.''

The political talk in town holds that a highly conservative justice, such as Reagan would prefer, might not be confirmable by a Democratic Senate. I'm not sure that's correct. If the Democrats come back, their margin will be narrow, probably skimpier than the present Republican advantage.

Should Reagan offer them any of a half-dozen well-qualified conservative jurists with experience on state or federal benches, it might be tough for the Democrats to turn him or her down. It would at least be politically risky; I can certainly see Republicans making an issue of such a vote in the 1988 campaign.

The vital question is whether Reagan will get an opportunity to appoint another justice or two to make the conservative bloc the court's majority. Justices William Brennan Jr. and Thurgood Marshall, the staunchest liberals on the court, are 80 and 77 respectively. Two others in the moderate center, Harry Blackmun and Lewis Powell Jr., are 77 and 78.

History and the actuarial tables favor Reagan's making another appointment. And last week's nominations make it clear that, like Plunkett of Tammany, he will know what to do with the opportunity.