President Reagan will try to capitalize on public support for fired national security aide Oliver L. North by asking Congress for increased long-term military aid for the Nicaraguan contras, White House officials said yesterday.

White House spokesman Marlin Fitzwater said Lt. Col. North's testimony had been "helpful" to administration efforts to build public support for the rebels opposing the leftist government of Nicaragua. Fitzwater said Reagan was considering asking Congress to fund the contras for an 18-month period extending into the next president's administration when the current appropriation runs out in October.

The exact amount of the request has not been determined, but administration officials said they were discussing a new request of about $140 million, up from a pending budget proposal of $105 million.

"The North testimony and the hearings in general appear to have opened a new window for advancing the cause of the contras," said a senior White House official who declined to be named. "It is not a groundswell that assures long-term support . . . , but it does give us an opportunity to make our case."

Reagan, who has tried for years with limited success to generate public support for the contras, is expected to make a new appeal after the Iran-contra hearings adjourn early in August. During a photo-taking session yesterday, the president urged continued aid for the contras and said, "A disinformation campaign by the Sandinista government has kept the American people from knowing a lot of the truth about the situation down there."

White House officials were buoyed by the findings of pollster Richard B. Wirthlin in an overnight survey of 600 persons, who were asked, "Would you support additional military aid to the contras who are fighting the Nicaraguan government?"

The poll, taken before yesterday's testimony, found that continued aid was favored by a narrow 48-to-46 percent margin. Statistically, this amounts to a dead heat, but it was the first time in any White House poll that a majority of Americans favored contra aid.

A senior White House official cautioned that "the challenge will be to keep the momentum for the next several weeks" before the administration's new contra aid proposal is submitted in September. Officials acknowledge that even Reagan, at the height of his popularity, did not succeed in building much public enthusiasm for the contras, but they believe that the Iran-contra hearings have focused unprecedented interest on the Nicaraguan conflict.

Conservatives who have been critical of congressional reluctance to aid the contras and impatient with administration efforts in their behalf tried to prod the White House into more dramatic action. Presidential candidate Rep. Jack Kemp (R-N.Y.) called on the Reagan administration to double its contra aid request to $210 million.

"The appearance of Oliver North before the Congress was far more than just political theater," Kemp said in a statement. "In six days he did more to build support for democracy in our hemisphere than the State Department and the Congress have done in six years."

Sen. Phil Gramm (R-Tex.) went even further, calling for a commitment of $1 billion to the contras that he said "could defeat the communist government in Nicaragua."

Even contra supporters have expressed doubts that the heavily armed and Soviet-backed Sandinista military forces will ever be defeated by the contras, who are basically a hit-and-run guerrilla army. But the White House tried yesterday to convey the impression that the contras' military prowess is improving.

Fitzwater said that 15,000 contras are operating within Nicaragua and showing "considerable success" in "achieving military goals."