(FORTUNE Magazine) – WHEN CONGRESS reconvenes in late January, a lot of well-heeled political action committees (PACs) will be targeting the Senate Finance Committee, which will decide the fate of the tax reform bill passed by the House in December. Business winners will fight to keep the breaks the House gave them; losers will try to plead their cases once more. A flood of PAC money is heading for the tax writers. A seat on the Senate Finance Committee, deadpans a committee staffer, ''does facilitate fund raising.'' Of the top ten PAC beneficiaries last year, four are members of the Senate Finance Committee. At the top of the list is the committee chairman, Bob Packwood (R-Oregon), who has raked in $812,273 -- more than any other legislator and more than half the amount the entire Senate Finance Committee raised in 1983. Republican members of the committee who, like Packwood, are up for reelection are twice blessed. Business lobbyists not only want the tax bill to go their way, but are also anxious to protect the Republican majority in the Senate. Says Bernadette Budde, a vice president of Bipac, a PAC that represents over 1,500 business associations and corporations, ''We want to make sure pro-business incumbents have large enough treasuries to scare opponents.'' Other Republican Finance Committee members running for reelection: Bob Dole (Kansas), Charles Grassley (Iowa), and Steve Symms (Idaho). Nearly 300 PACs have contributed to the Symms campaign, including Boise Cascade, Champion International, and Weyerhaeuser. The paper companies hope the first-term Senator can help restore the tax benefits on timber sales, which were lost in the House version of tax reform. The National Association of Realtors, a top PAC spender every year, devoted 12% of its 1985 PAC budget to members of the House Ways and Means Committee and the Senate Finance Committee, vs. about 7% in 1983-84. The House retained deductions for property taxes and mortgage interest on second homes. To encourage the Senate to follow along, the real estate lobby has already provided $14,000 to Finance Committee members, including Dole and Packwood, and is likely to more than double that amount this year. Insurance companies also gave big and won big. Three major life insurance associations forked over some $250,000 in PAC money to legislators wrestling with the tax bill last year. One of them, the National Association of Life Underwriters, delivered 34% of its PAC money to tax writers, nearly three times as much as in 1983-84. In the House the industry was successful in killing a proposal to tax so-called inside buildup, or the appreciation in the value of life insurance policies. To keep that proposal buried and to push other pet issues, insurance PAC officials say they'll continue to funnel funds to the Senate Finance Committee. Defense contractors are among the losers who hope to make out better in the Senate. The House eliminated ''completed contract accounting,'' which allows contractors to defer income, often for a decade or more. Big banks are looking for a restoration of deductions for bad-debt reserves and interest on money borrowed to buy certain tax-free securities. Oil and gas lobbyists will fight House tax provisions that would cost the industry some $4 billion over five years. Small PACs are getting into the act too. The American Horse Council Inc., a group of horse owners and breeders, kicked in $4,500 to Dole, Grassley, Packwood, and Symms. The Horse Council wants to depreciate horses more rapidly than the House bill would allow. Lawmakers are stepping up the pressure for more funds. Legislators, including tax writers, call and say, ''I'm dealing with an issue very close to your heart,'' according to Mary Hasenfus, director of political affairs for the U.S. Chamber of Commerce. Says she, ''It's getting very close to extortion.'' One Senate Finance Committee member, David Boren (D-Oklahoma), isn't making calls. Boren doesn't accept PAC funds because he says the ^ practice looks like vote selling. Etta Fielek, Packwood's communications director, points out that her boss's PAC money makes up only about 15% of his war chest. Says Fielek, ''It's naive to assume that before a vote a U.S. Senator sits at his desk and reads a printout of which PACs favor him and which don't.'' Boren authored a bill to limit PAC spending last year. The bill got nowhere.