Topic A, even among those politicians whose attention span for economic issues is no more than 30 seconds, is whether or not President Reagan will reappoint Paul A. Volcker as chairman of the Federal Reserve Board.

After all, according to most polls, Volcker is rated the national figure second in power to the president of the United States because he manages the independent central bank of the biggest nation in the world.

There are many who believe--in the words of Nobel Prize winner James Tobin--that "the Federal Reserve . . . is the last citadel protecting the dollar and the country from disaster."

But Volcker, who has had a high visibility on the international economic scene, will be out of the chairmanship early in August, unless Reagan asks him to stay on.

The White House, which has been stingy with its praise for Volcker, has been following an open-mouth policy leading to a stream of inspired stories suggesting that Volcker is unlikely to be reappointed.

Variously, such stories say that the president is entitled to "his own man" at the Fed; or that Fed policy under Volcker has been too erratic; or that Fed policy may soon have to be more "flexible" to permit sound economic recovery than Volcker, who is too independent, may allow; or that there are plenty of others who can do just as good a job--if not better.

The cynical among us believe that at the root of the problem is a presidential conviction that the inflation problem is licked, that the economy is on the way up--and that there is no need to share the political benefits of such good results with someone appointed by Jimmy Carter.

But as The Economist last week shrewdly observed, if the big Reagan deficits precipitate higher interest rates this winter, "Mr. Reagan will take more blame with Mr. Volcker gone" because money policy will then be in the hands of his own man.

Yet, here and in financial markets in New York, almost everyone is convinced that Reagan has already decided to replace Volcker with someone else, even if the new man's economic philosophy and direction of the central bank would be very close to Volcker's.

"Unfortunately," said New York economist Henry Kaufman, "I sense the process of conditioning the financial markets to a change." Every time the money supply increases, Treasury officials wind up and take a pot shot at Volcker.

The most convincing sign that Volcker is probably finished is that the economic summit has come and gone with no announcement that he will stay: had Reagan decided to continue with Volcker, the summit would have provided a convenient backdrop for the announcement, because it surely would have been warmly received--even praised--by America's summit partners.

The most plausible replacement for Volcker is former president Ford's chief economic adviser, Alan Greenspan, who isn't seeking the job--but who wouldn't turn it down, authoritative sources tell this reporter. Greenspan, who has been urging Volcker's reappointment (as do most prominent financial analysts and businessmen), would not depart significantly from Volcker's approach.

Others prominently mentioned include Fed Vice Chairman Preston Martin, a Californian named to the board 18 months ago by Reagan; the father of monetarism, Milton Friedman; Citicorp Chairman Walter Wriston, an ideological monetarist who gets along well with Reagan; and the pragmatic Treasury Secretary Donald T. Regan, who could well conclude that these days, the Fed job is more important than the Treasury assignment.

But a Reagan decision to let Volcker go, hoping to assure that there is an "accommodative" Fed chairman in the election year of 1984--should the economy need a lift--could easily backfire.

To erase any suspicion that he is knuckling under to Reagan's political orders, a new chairman--and his fellow board members--would have to bend over backwards, and monetary policy therefore might be more restrictive than it otherwise would be in 1984.

"If you think that financial markets are sort of paranoid now, watching the money supply figures like a hawk, wait until Paul goes," said a key analyst close to the administration. "If they have to, they'll devise an hourly money-growth report!"

Or as Kaufman put it less dramatically: "A new chairman, in order to perform successfully, will have to demonstrate very early that he is independent, or financial markets will become apprehensive.

"In that sense, a new chairman would have less flexibility than the current chairman."

What Kaufman and others are saying is that, ironically, it is possible that Volcker--whose independence has been so well certified by Reagan--would be able to follow an easier money policy in 1984, if the circumstances demanded it, than Reagan's "own man," who might find it necessary to demonstrate that he's no one's lackey.

Here's another risk the president faces in replacing Volcker: if he appoints a chairman considered weak by the other 11 members of the Federal Open Market Committee (FOMC)--the key policy board of the system--he could have a revolution on his hands.

It is true that the Fed chairman has impressive powers: he controls the Fed staff, he deals with the other central banks and he is viewed as the chief internal persuader, spokesman for and defender of the system.

But the Fed chairman has only one vote and he must be able to motivate his six fellow governors on the Fed board and the five regional bank presidents who, with the seven-man board, make up the FOMC.

To be sure, there have been some extraordinarily powerful and charismatic recent chairmen of the Fed, including William McChesney Martin, Arthur F. Burns, and Volcker. But that doesn't mean that any chairman automatically will wield the kind of power those three did.