These first nights of autumn are rough ones for the well-heeled lobbyist. It seems every Democrat in town is after his wallet.

Take Sept. 24, for example. The day started with a $100-a-head fund-raising breakfast for Rep. Ike Andrews (D-Calif.). Then, Rep. Gillis W. Long (D-La.), who won his last election with a mere 69 percent of the vote, upped the ante considerably by charging $500 per for a poolside fund-raiser at the Watergate.

Rep. Don Edwards, the fourth-ranking Democrat on the House Judiciary Committee, weighed in by asking contributions of from $50 to $250 at the Folger Shakespeare Library. Rep. Bob Traxler (D-Mich.) asked $250 for another affair on Capitol Hill.

And if all that wasn't enough, Rep. Cardiss Collins (D-Ill.) wanted $150 for a boat ride down the Potomac.

Although the next general election is more than a year away, such days are not uncommon on the Washington political circuit, once relatively quiet during off-election years. "People can't do all the functions they'd like to," complains one lobbyist. "Simple logistics make it impossible."

The proliferation of the Washington fund-raiser as a political institution has been particularly dramatic among Democratic congressmen. They have started raising money earlier and harder than ever before. During the last two weeks of September alone, they held 26 Washington fund-raisers.

The reason?

"It's a four letter word--FEAR," says Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee.

Fear of President Reagan, fear of reapportionment, fear of New Right political action committees, and fear of the Republican Party's fund-raising apparatus, which outdid the Democrats 10 to 1 in 1980, and 12 to 1 the first half of this year.

"It's not so much that Democrats are scared," says one lobbyist. "It's just a fear of the unknown. And the unknown is a big dollar sign."

Dozens of senior Democrats, who have never felt threatened before, are trying their best to build up campaign war chests. In his first 32 years in Congress, for example, Rep. Charles E. Bennett (D-Fla.), who won his last race with 77 per cent of the vote, never found it necessary to hold a single event to raise money.

This year he changed his mind when an old friend told him he would face "stiff opposition" in 1982 if he voted against Reagan's tax cut, which he did. The message from the rich friend was clear. "It made me realize the wealth in my district would go against me," Bennett says.

So Bennett started a modest fund-raising effort. He insists he didn't do so because he feels endangered. He says he just wants money to buy enough television ads "to let people know I'm still interested in the job."

He is hardly alone.

Rep. Joseph P. Addabbo, an 11-term New York Democrat who carried 95 percent of the vote during the last election, is worried enough about the next one to salt away $95,231 for it. House Budget Committee Chairman James R. Jones (D-Okla.) has put away $131,091, and scheduled his second Washington fund-raiser of the year, a $250-a-head affair to be held Thursday.

Rep. Dan Rostenkowski, chairman of the powerful House Ways and Means Committee, recently decided that he, too, would change his ways and hold a fund-raiser. "Dan Rostenkowski has spent more years in Washington without holding a Washington fund-raiser than any other U.S. Congressman in history," the glossy invitation boasted. "Be there to help break the record!"

Each of the above has a slightly different reason for the early fund-raising start. Jones, for example, has been targeted by the National Conservative Political Action Committee, which has already begun a campaign against him.

Rostenkowski also has been targeted by NCPAC, but he holds a safe Democratic seat in Chicago. His chairmanship of the tax writing committee makes him a big draw on the money circuit, and he has no compunction against using that position to further his leadership ambitions in the House. According to aides, he is raising money for himself this year so he can be free to help others in 1982.

The big uncertainty for Addabbo is reapportionment. "We're going to lose four seats in New York City," says an Addabbo spokesman. "Someone is going to get hurt bad, but we won't know who until almost January. You have to be prepared."

Traditionally, Democratic congressional candidates have held a slight edge nationwide in fund raising. As individuals, they still hold their own with Republicans. But the GOP has built up an extremely effective fund-raising apparatus in recent years, capable of funneling tens of thousands of dollars into congressional races.

During the first six months of this year, the Republican National Committee and the party's House and Senate campaign committees raised $36.7 million, compared with only $2.8 million for their Democratic counterparts.

These figures send chills up the spines of Democrats. They know Republicans and conservative political action committees will have money to experiment with in 1982. Seats that have long been thought "safe" may no longer be so. In reaction, many Democrats have begun year-around fund raising.

"I intend to get over $300,000 by the end of this year," says Democrat congressional campaign chairman Coelho. "I assume they will target me and I'm going to be ready."

Another unconfirmed, but vivid, fear should be noted here. "The word on the street is that the White House is going to make 1982 a test of manhood for their big money friends. That means no money for the 'Ds,' " says a lobbyist with a major Washington firm. "The reaction from a lot of congressmen is we better raise money in '81 because the well will be dry in '82."

With the growth in number of lobbyists and political action committees during the last decade, Washington fund-raising events have became a staple in the political money game. A typical congressman may pick up $25,000 during one such event, a well-placed committee chairman $75,000 or more.

But some feel the market is oversold. "Washington is beginning to be viewed as a money mecca, and it's overrated," says one Democratic strategist.