President Carter, showing no sign of second thoughts, yesterday strongly defended the ouster of David W. Marston as the U.S. attorney in Philadelphia and said of his own intervention in the case: "If it occured now, I would do the same."

Clearly prepared for questions about the Marston case, the president told a nationally televised news conference that he sees no conflict between the firing of the Republican prosecutor and his campaign pledge to remove the Justice Department from politics.

"I see nothing improper in the handling of the case," he said. "I made a campaign commitment that any appointee to a position as U.S. attorney or a judgeship would be appointed on the basic of merit, and this campaign commitment will be carried out. . . . I think the attorney general has handled the case as well as possible."

Marston has prosecuted a number of Democratic political figures in the Philadelphia area. At the time of his ouster, made final Jan. 20 by Attorney General Criffin B.Bell, he was investigating the construction of a $65 million addition to Philadelphia's Hahnemann Hospital.

The inquiry has given rise to allegations that two Democratic congressmen from Phiadelphia, Reps, Deniel J. Flood and Joshua Eliberg, may have project.

Carter became embroiled in the controversy at a news conference Jan. 12 when he admitted that last No- vember Eilberg called and asked him to "expedite" the replacement of Marston. He said he passed that request on to Bell.

The president has consitently asserted that at the time of the telephone call he did not know that Eilberg figured in the Marston probe. "As far as any investigation of members of Congress, however, I am not familiar with that at all and it was never mentioned to me." he said on Jan. 12.

Later, however, Carter signed a statement, part of a Justice Department inquiry into the Marston ouster, in which he said he first learned that Eilberg was of "investigative interest" in Philadelphia just before his Jan. 12 news conference.

The president said yesterday his Jan. 12 statement had to do with what he knew at the time of the November telephone call, and not what he had learned just before the news conference.

Defending both the firing and his role in the case, he said. "Yes, I think that our actions are compatible with my campaign statments . . . This was a routine matter for me and I did not consider my taking the telephone call from Congressman Eilberg nor relaying his request to the attorney general, to be ill-advised at all. If it occurred now, I would do the same."

Bell today will interview two potential replacements for Marston - sam Dash, the Senate Watergate committee counsel and Clayton Undercoiled III. Marston's predecessor as U.S. attorney in Philadelphia.

Meanwhile Senate Minority Leader Howard H Baker Jr. (R-Tenn.) said he will press for further investigations into Marston's ouster, "I am not at all convinced that a thorough, detailed and satisfactory inquiry has been completed" Baker said.

The Marston case was the dominant topic at yesterday's exceptionally wide-ranging news conference.

In response to one question. Carter denied a published report that the Soviet Union has or will soon have the capability to disrupt the sending of military orders by satelite. However, the denial may have been hinged on a technicality.

Defense Secretary Harold Brown told a Pentagon press conference last Oct. 4 that the Soviets had an operational satellite killer that could hit "some" U.S. satehites. The "some" is believed to be low-flying reconnaissance satellites, not the communication satellites that hover 25,400 miles above earth.

ON other topics, the president:

Opposed the demands of militant farmers for full parity on tarm products, arguing that this would cost up to $25 billion and make America farm products noncompetitive in international markets. With full parity, the government would raise support prices so that they equaled in purchasing power what they were in the legally fixed base period of 1910-1914.

Said the administration has no plants to introduce a bracero-type program of using Mexican workers.

Said he "deplores" a planned march by Nazi Party members through the heavily Jewish city of Skokie, III, but that attempts to halt the march are best left to the courts.