President Carter, confronted by mounting economic problems that could affect his reelection, moved yesterday to tighten up the administration's economic policymaking machinery.

Carter named Treasury Secretary W. Michael Blumenthal as the administration's chief spokesman on all economic issues and directed that all officials clear major policy statements through the Economic Policy Group's "Steering Group," which Blumenthal heads.

In a memo sent to the heads of executive departments and agencies and the White House senior staff, Carter also made it clear that the EPG's Steering Group will be in complete charge of policymaking.

Under the arrangment, the steering Group - consisting of Blumenthal; Charles Schultze, chairman of the Council of Economic Advisers; James McIntyre, director of the Office of Management and Budget, and Alfred Kahn, presidential adviser on inflation - will meet several times a week in the White House, with Carter attending at least once a week.

Carter had said last winter that Blumenthal was the administration spokesman on the economy and Kahn the spokesman on inflation.

But Kahn recently has angered the White House with his outspokenness, which culminated this week with a suggestion that the president should have vetoed last year's $18.7-billion tax cut legislation.

Vice President Mondale and Stuart Eizenstat, presidential assistant for domestic affairs and policy, will continue to participate in all meetings of the Steering Group.

The memo's wording makes it plain, sources close to the White House said, that neither Labor Secretary Ray Marshall nor Commerce Secretary Juanita Kreps normally will participate in Steering Group deliberations. Both have been pressing for more liberal options on recent policy decisions.

The status of Undersecretary of State Richard Cooper, who normally has been attending the weekly breakfast meetings of the Steering Group of the Treasury Department, is less clear. Blumenthal is comfortable working with him and might continue to invite him, sources speculated.

Carter not only explicitly narrowed the size of the Steering Group, he also broadened its power.

The memo said the EPG "shall have an office in the White House . . . [and] . . . shall have full access to all decision memoranda - from agencies and from [the Executive Office of the President] and White House staff units - which involve policy issues having a significant impact" on the economy.

That will mean the Steering Group will get a look at just about everything and, through the weekly session with Carter, pass its views directly to the president.

One problem the memo is intended to correct, sources indicated, was that too many pieces of paper on economic matters were reaching the president through too many channels. No one was relating issues in one area.

In appearance and in reality, decisions were being made without an adequate attempt to reconcile competing goals. This sometimes left Congress and the public confused as to what policy really was, sources said.

Within the Steering Group itself there was uncertainty about exactly what it should be handling and what Carter wanted it to do. Its meetings often touched on a variety of subjects and reached decisions on none, the sources said.

Apparently there now will be more frequent meetings devoted to single subjects, with the results going quickly to Carter.

"This really crowns Blumenthal," said one observer. "There have been too many people making statements, and maybe this will help."

Blumenthal, according to one report, asked Carter to restate the Steering Group's mission, and the memo, which was released with no fanfare or even explanation, is the result. It had been "in the mill" for at least three or four weeks.