President Reagan mixed pleasure and politics here today on his first presidential visit to one of the troubled urban areas that stand to lose the most from his reductions in federal spending.

Mayor Edward Koch estimates New York City's loss under Reagan's proposals at $4.4 billion over the next three years.

President and Mrs. Reagan are spending the weekend in the $1,700-a-night president's suite at the Waldorf Towers, and Reagan's weekend schedule gives the impression that this visit is primarily a luxurious holiday. s

The Reagans are attending the Broadway musical "sugar Babies" tonight and having a private dinner at an elegant French restaurant.

On Sunday, the Reagans will leave their 10-room suite only once, to watch their son, Ronald P. Reagan, dance with the Joffrey Ballet.

The White House says the trip is for political purposes and the Republican National Committee is paying the entire cost.

One political achievement of the weekend was the persuasion of Rep. Guy Molinari (R-N.Y.), a sort of congressional Rodney Dangerfield, not to resign from Congress.

Molinari, a freshman, had complained of being ignored in Washington and threatened to take a judgeship in his native Staten Island.

Rather than see the Reagan Republican resign, Sen. Alfonse D'Amato (R-N.Y.) and the White House moved to make Molinari feel more respected and appreciated.

Molinari was added to the luncheon guest for today in New York's Little Italy district, and he took the occasion to announce: "I'm staying on in Congress."

Reagan hailed the decision. "Congressman Molinari has made a very great personal sacrifice to stay and help up with this program that we are trying to implement," he said.

About 400 people crowded onto Mulberry Street outside Angelo's Restaurant in Little Italy to hear Reagan's brief remarks.

D'Amato told the crowd that Reagan had come "to say he loves Little Italy." The freshman senator added that Reagan's programs are in the best interest of Little Italy -- which brought a shout of "bull" from a young man watching the event from a fire escape almost directly above Reagan's head.

The president's other political sessions were an hour-long meeting with Mayor Koch and a visit to a New York State Republican leadership reception that was held in the Waldorf.

Although Koch is a Democrat, the Reagan White House considers that, as a senior aide said, he was "not unfriendly" during the election campaign.

Koch said after the meeting, "I like him. I think he's a man of character." The mayor said that he presented Reagan with alternative proposals for controlling government spending that would inflict less damage on New York.He was promised a meeting with Office of Management and Budget Director David A. Stockman next week to discuss these proposals and Koch's opposition to reducing subsidies for mass transit.

Reagan was asked whether this was a fence-mending trip. "No, I didn't think there were any fences here to be mended. I came up here to -- Nancy had some things to do here, things she's interested in, drug programs and so forth. And while we're going to have some entertainment, why, this is, I think, business," he said.

Nancy Reagan discussed drug abuse with Cardinal Terence Cooke Friday. No other durg talks were on her schedule.

She did not go to Little Italy today, deputy White House chief of staff Michael Deaver said, because she stayed in the Waldorf Towers working on revisions of her autobiography, which is being reissued in paperback.

A representative of the Waldorf said that every president since Herbert Hoover in 1931 has stayed in the four-bedroom president's suite with its four marble bathrooms.