What is a mild-mannered U.S. senator from Colorado doing in this flashy New York apartment with imported French wood paneling, surrounded by Men in custom-made shirts and glamorous women who look like Charlotte Ford? Why all the Secret Service agents out front on Fifth Avenue?

Floyd K. Haskell (D-Colo.) is quick to admit he doesn't like it. Nor does he see how to avoid it. "I have a terrible feeling that I'm going to have to do this right along until November," he admits.

Do what?

Look for money -- more than half a million dollars, the minimum the Haskell reelection campaign feels is needed to renew Haskell's mandate from the people of Colorado.

To raise that money, Haskell must rely on a number of Senate colleagues, administration officials and political allies, some of whom he barely knows, to grease the machinery of America's political money machines. It isn't fun, and there isn't any choice.

On this night the need for campaign money has brought Haskell to the beautiful apartment of Edward Downe Jr., who made a fortune publishing magazines including the Ladies' Home Journal, then sold out a few years ago and lives on the proceeds. Downe and several other New Yorkers, whose shoes combine with enough else to qualify them as well-heeled, have organized a cocktail party for Haskell featuring Vice President Mondale.

On the shuttle from Washington, Haskell acknowledged that he didn't know Downe, nor most of the people who were invited to the reception. This is how the game is played, Haskell said fatalistically. The man who arranged it all was Howard Samuels.

Howie the Horse - a New York perennial. He got his nickname on the Off-Track Betting board. He was a county executive on Long Island, ran unsuccessfully for governor, and has been around the political track a hundred times. A millionaire and management consultant, Samuels now keeps a hand in by helping other politicians raise money.

Haskell's finance chairman, a Denver real estate man named Chat Paterson, approached Samuels last year and asked if he could help the senator's reelection campaign. Samuels met the senator. "I like him," he said later."I became convinced his race was one of the most important campaigns in the country this year." Moreover, the two men were a good match on the tennis court. A political relationship was born.

This was the second New york fund-raiser Samuels had set up for Haskell. Mondale was fashionably late; the crowd was fashionably deployed on Downe's French antique furniture. One of the women who looked like Charlotte Ford turned out to be Charlotte Ford - a friend of Downe's. Young men in tuxedos passed among the crowd with huge drinks and elaborate hors d'oeuvres.

Ransom conversation revealed that virtually no one in the room know much about Haskell, a senator who seems constitutionally incapable of promoting himself. People came because they were invited - in a society where one's own invitations get accepted if one accepts those of others.

"Ed (Downe) gives to my candidates and I try to give to his," said one of the guests, a political appointee in the Carter administration who asked not to be named. "I live in the building, Ed's a friend of mine," said Martin Revson, brother of the late Charles Revson, the Revlon tycoon.

Downe himself said he was cochairman of the finance committee in Gov. Hugh Carey's reelection campaign, and agreed to hold the party as a favor to Samuels and others involved. The men in tuxedos, their drinks and canapes cost nearly $1,000 - Downe's contribution.

Stephen Greenber, a Wall Street whiz who wears his gray hair Ben Franklin style, to the shoulders, assured friends that Haskell was okay - he voted right on Israel. (The day before, Haskell voted with friends of Israel against the Carter administration's plan to sell warplanes to Egypt and Saudi Arabia as well as Israel.)

But most of the guests knew little about Haskell, and many asked a visitor from Washington what he was like as a senator.

Mondale arrived, surrounded by a bustle of agents and reporters who had been following him aroung New York. It was his third fund-raiser of the evening - he'd already spoken up for Rep. Stephen J. Solarz (D-N.Y.) and Sen. Wendell Anderson (D-Minn.) at other fancy gatherings in Manhattan.

The vice president shook hands with everyone in the room, then made a few remarks. He recalled that as a senator he had fought for years for decent housing for low-income Americans. "And it's not bad when they put it up, is it?" Mondale asked, looking aroung Downe's apartment.

A recent story in The Washington Post had quoted Haskell as attacking the Carter administration's secretaries of agriculture and interior as the managers of "hated" policies in Colorade, apparently a misquote based on a misunderstanding between the senator's press secretary and a reporter. Mondale took up the jibe nevertheless, and observed:

"We have a deal. Floyd attacks Carter and Mondale, and we support Haskell as "a gifted senator," a "man of conscience" and the like, and that was it.

Haskell then made a few - very few - remarks, thanking Mondale, assuring him profusely that he thought the secretaries of agriculture and interior were fine fellows, and assuring the crowd that he would wage a tough campaign against a Republican candidate who was certain to be financed by generous contributions from right-wing groups.

Then Samuels spoke up, announcing several $1,000 contributions, and asking bluntly for more. By the end of the evening, the Haskell campaign had pledges of more than $20,000. A campaign aide said that a series of New York fund-raisers should produce at least $75,000 by November.

Out-of-state help is a common feature of Senate campaigns. A substantial portion of Haskell's campaign treasury thus far has come from Louisiana - largely from executives of oil firms in that state.

Why? Because Sen. J. Bennett Johnson (D-La.) is Haskell's closest personal friend in the Senate, and has been raising money for him. Sen. Russell B. Long (D-La.), Johnston's colleague and chairman of the Finance Committee on which Haskell serves, has also helped.

Johnston acknowledges that Haskell's moderate-to-liberal voting record is substantially to the left of his own, "but there are just some people you like." (The two men regularly play tennis together.)

Though Haskell has taken public positions on energy-related issues that have infuriated many segments of the oil industry, Johnston said he could convince Louisiana oil executives to contribute to his friend's campaign with the argument that he was an honest, open-minded senator, "and when I really need him, he'll be there. They said, "that's good enough for me," Johnston said in a recent conversation.

Sen. Edward M. Kennedy (D-Mass.) came to Colorado this spring to raise money for Haskell and help him get some publicity in the Colorado media. Sens. Edmund S. Muskie (D-Maine), Williams (D-N.J.) and Robert C. Byrd (D-W.Va.) have also lent their presence to Haskell fund-raisers.

If there is any pleasant aspect to all this for Haskell, he says, it is the readiness of colleagues to help out. When he asks, they respond affirmatively, he says. "I said to Scoop (Sen. Jackson), 'Could you come up to New York for a fund-raiser?' And he said he could."