Rep. Frank J. Guarini (D-N.J.) holds no leadership post, ranks behind half the House in seniority and is scarcely known outside his state. So why, when most House members are holding $250-a-ticket fund-raisers, is Guarini charging $1,000 -- and getting it?

The answer, according to aides and lobbyists, is that Guarini sits on the tax-writing House Ways and Means Committee, always a magnet for political contributions, more so this year as Washington braces for the most serious effort to overhaul the federal tax code in years.

"So many different industries and groups stand to be affected by this, and there's a lot of anxiety," a Guarini aide said. "When there's that much concern, the opportunities for fund-raising increase. People want to have access. They want to put in their two cents. Or their thousand dollars."

President Reagan is to unveil a proposal on May 28 to "simplify" the federal tax code, wiping out or limiting many lucrative write-offs and credits, while lowering tax rates across the board.

No other Reagan proposal has more potential to affect business profits, since many industries -- oil and gas, real estate, insurance, timber and more -- are built around tax breaks.

Ways and Means and the Senate Finance Committee will sit in judgment on the proposal, and for months, business and labor lobbyists have been vying for their members' ears. It is standard practice for political action committees to contribute more money to members of those panels than others, but this year, several committee members said the discrepancy is even greater.

"This is such a broad-based proposal," said Rep. Robert T. Matsui (D-Calif.). "There are many more lobbyists coming to fund-raisers as a result. There's just no question."

Guarini and others are holding fund-raisers with fewer than 30 lobbyists at a time, offering more access than a large event, and charging for it. Guarini, who grossed $27,000 at a $1,000-a-plate breakfast at the Hay Adams Hotel on May 1, plans three more, according to an aide. He also plans a larger fund-raiser at $500 a ticket, or twice the going rate for House members.

Rep. Ronnie G. Flippo (D-Ala.), who is newer to Ways and Means than Guarini, has held two small $1,000-a-plate dinners since April at the Sheraton-Carlton Hotel and The George Town Club, hosted by lobbyists from home-state steel and energy companies that depend heavily on tax subsidies. He also held a large fund-raiser in March at $500 a ticket, which grossed $70,000, according to an aide.

Rep. W. Henson Moore (R-La.) is using the cachet to help build a warchest for an expensive 1986 race to replace retiring Sen. Russell B. Long (D-La.). Moore, who had $725,000 left over from the 1984 campaign, plans 10 small $1,000-a-plate events this spring, and held a $1,000-a-couple banquet in Baton Rouge this weekend that was expected to raise $400,000.

"The connection might be drawn that there's a tax bill coming up and Ways and Means members are all having fund-raisers before it comes to a vote," said Rep. Thomas J. Downey (D-N.Y.). "Is this the reason I'm having a fund-raiser? No. Does it mean more people will show up? The answer is probably a lot more."

Among the biggest beneficiaries is Sen. Bob Packwood (R-Ore.), chairman of the Finance Committee, who faces reelection in 1986. He has raised more than $1.4 million this year -- more than he spent in 1980 -- at large fund-raisers in Portland, Washington, New York, Chicago and Dallas, hosted by real estate developers, oil company executives, securities brokers, lawyers, utility executives and timber company officials.

"Raising money for Bob Packwood is like making love to a 700-pound gorilla. It doesn't make a difference when you get tired," Lester Pollack, a longtime Packwood friend, observed at the New York fund-raiser.

Ways and Means Chairman Dan Rostenkowsi (D-Ill.) plans a $500-a-person party next month for his Chicago Campaign Committee, which donates to other House members' campaigns. But he has scheduled none for himself this year.

The tax-writing members of Congress may not be holding more events than others, but they appear to be luring larger contributions.

Democratic House members other than those on Ways and Means are scheduled to hold 91 fund-raisers by November. Of those, 12 -- or 13 percent -- are charging $500 a ticket. However, of eight Ways and Means Democrats scheduled to hold large fund-raisers, six -- or 75 percent -- are charging $500.

Matsui is holding a $300-a-ticket fund-raiser next week, which a prominent Washington lobbyist described as "bargain basement" for Ways and Means.

The Washington fund-raisers are attended largely by lobbyists for political action committees (PACs), who say the contributions are aimed not at swaying votes, but at aiding the reelection of friendly members of Congress. They said they would give the same amount to tax committee members even if the simplification effort were not afoot. However, many said they view the donations as a payment for access to members. And this year, access is even more important than usual.

Corporate PACs are also contributing heavily to tax-writers. Rep. Hal Daub (R-Neb.), new this year to Ways and Means, received $1,000 from a real estate PAC, which is fighting to save real estate tax deductions. The PAC gave the same amount to such influential House members as Budget Committee Chairman William H. Gray III and Judiciary Chairman Peter W. Rodino Jr. (D-N.J.), according to Federal Election Commission records.

The PAC for General Electric, which paid no taxes from 1981 to 1983 despite $6.5 billion in profits thanks to assorted tax breaks, gave $2,050 to Packwood through March. The National Association of Life Underwriters PAC gave him $3,000, FEC records show. The United Auto Workers has given more to Ways and Means members than most of their House counterparts.

The outpouring of funds has prompted charges that the tax-revision effort, aimed at reducing the influence of special interests over tax policy, instead could strengthen it.

"The play-out of this tax fight will demonstrate to all of us just what an enormous national problem we have today in the financing of congressional elections," said Common Cause President Fred Wertheimer.

But most business groups and legislators see it differently.

"On a matter of national public policy like taxes, giving contributions isn't going to win one damned thing," said attorney Edward Forgotson, a GOP fund-raiser and lobbyist for Texas Oil and Gas Corp. ". . . You can't make those kind of deals. If you try, you're going to be throwing your money away."