The Carter administration asked Congress yesterday for legislation aimed at reducing the number of lawsuits heard in the U.S. District Courts by giving federal magistrates vastly expanded powers to decide civil and criminal cases.

At a press conference, Attorney General Griffin B. Bell said the proposed bill, which could lead to as many as 16,000 cases a year being shifted from judges to magistrates, would greatly improve the public's ability to get cases into court and have them decided quickly.

The practical effect of the bill would enable magistrates to decide virtually all cases in the jurisdiction of the federal courts - provided that all parties to the dispute agree to trial before a magistrate rather than a district judge.

This requirement - that plaintiffs and defendants consent to trial by a magistrate - represents an attempt by the administration and by Sen. Dennis DeConcini (D-Ariz.) who introduced the bill in the Senate yesterday to disarm potential opposition to the legislation.

As originally proposed by the Justice Department, the legislation would have made mandatory the transfer to magistrates' jurisdiction of approximately 12,000 suits involving claims under the Social Security and black-lung compensation laws.

That had aroused opposition from poverty-law forces, who charged that the poor and elderly would be relegated to a system of "second-class justice" where they would be given inferior quality hearings and have their appeals made more difficult.In particular, the opponents argued that the legal qualifications and abilities of many federal magistrates are questionable.

To overcome these objections, Bell said yesterday, the Justice Department was "satisfied to go along with the consent approach in all cases except minor criminal offenses such as speeding on a government reservation." He noted that Thomas Ehrlich, president of the Legal Services Corp., the government-funded but legally independent manager of federal poverty law programs, has said that he will now testify in favor of the bill.

Underlying the proposal is concern about the growing backlog of unresolved cases in the U.S. District Courts, where new cases are being filed at the rate of 160,000 a year. The latest figures compiled by the Administrative Office of the United States Courts show a backlog of 365 pending cases for each of the 398 authorized District Court judgeships.

The administration's bill calls for easing this load by making greater use of the 164 full-time magistrates and 323 part-time magistrates already in the federal court system. They are lawyers appointed by the district judges to assist with such chores as holding preliminary hearings for persons charged with a federal crime or conducting fact-finding hearings in cases before they go to trial.

Passage of the bill theoretically would enable them to deal with almost the full range of federal court cases. As a practical matter, though, Justice Department officials say the system would most likely be used by litigants in civil cases involving relatively small amounts of money or defendants on charges carrying relatively minor penalties.