The debate over the reappointment of Paul A. Volcker as chairman of the Federal Reserve Board picked up steam yesterday as two key Senate Republicans sang Volcker's praise and a leading House Democrat attacked him.

Sen. Jake Garn (R-Utah), chairman of the Senate Banking Committee, gave tentative backing yesterday to the reappointment of Volcker as Fed chairman when his term expires in August.

"I wonder, if he is not reappointed, what kind of signals do you send to the marketplace," Garn told a meeting of the U.S. League of Savings Institutions.

Later in the day, Sen. Robert J. Dole (R-Kan.), chairman of the Senate Finance Committee, said Volcker "has been doing an outstanding job" as chairman.

In remarks to the Public Securities Association in Washington, Dole said Volcker is the only one in Washington who has been "holding back the floodgates" on the money supply.

From the House side, however, Banking Committee Chairman Fernand St Germain (D-R.I.) sharply criticized Volcker for "joining" the Shadow Open Market Committee in suggesting a possible slowdown of money growth.

Referring to Volcker's testimony Tuesday before the House Budget Committee, St Germain said the Fed chairman's remarks sounded "all too much like an echo of the Shadow Open Market Committee's call for quick return to the discredited policies of monetarism."

The shadow committee is a group formed 10 years ago that offers unofficial advice from a monetarist viewpoint to the Fed's policymaking Federal Open Market Committee.

President Reagan, who would have to reappoint the 55-year-old banker to a new four-year term as chairman, has not said what he plans to do. The nomination would then have to be confirmed by the Senate.

Volcker, former president of the Federal Reserve Bank of New York, was named Fed chairman in 1979 by President Carter.

Volcker's tenure as head of the nation's central bank has been controversial, marked by an economy that has undergone two recessions and interest rates that have soared to record levels.

Even so, he enjoys support among both Republicans and Democrats and the respect of the financial community for having the tenacity to bring the inflation rate down sharply.

With the economy now in the early stages of a recovery, politicians have been saying Volcker becomes a more appealing choice for another term as chairman.

Garn, in his remarks, said, "my inclination would be the best course not to send signals is to keep him Volcker there."

He praised Volcker as having done "a rather remarkable job," given the "difficult circumstances imposed on him . . . by a fiscally irresponsible Congress."

"I don't know anybody who could have done a better job," he added.

Garn said that if Volcker were replaced as Fed chairman, "you'd better put somebody in there who's perceived as tough and willing to stand up to the guff. You don't want to open up the money markets."