Four months ago it seemed as if any tax increase this year would fall mainly on businesses rather than individuals, but now, as the Senate Finance Committee prepares to crank out a $20.9 billion tax bill, it seems the other way around.

Corporate lobbyists have made important progress in the Finance Committee, many of whose members must stand this year for reelection.

Those seeking to save last year's tax leasing provision, under which companies can now buy and sell tax breaks, have won an ally in Sen. David Durenberger (R-Minn.), whose constituents include big railroads and Republic Airlines, major beneficiaries.

Sen. John C. Danforth (R-Mo.) is sponsoring a bill that would allow defense and other contractors to retain a tax deferral provision that the administration wants to end, and he expects strong support from his colleagues.

The energy state senators--Malcolm Wallop (R-Wyo.), David L. Boren (D-Okla.) and Lloyd Bensten (D-Tex.), to name just a few--are convinced that the administration's proposed minimum tax on corporations would be disastrous for the oil industry, and they are fighting it.

Sen. John H. Chafee (R-R.I.), meanwhile, has joined Bensten and others in sponsoring legislation drawn up by the insurance industry to counter an administration proposal that would eliminate a special insurance tax break.

Even Sen. Robert J. Dole (R-Kan.), the committee chairman and self-proclaimed leader of a drive for "fairness and equity," is now arguing that the cornerstone of this drive, the corporate minimum tax, has an "unduly harsh impact on a few particular industries," notably independent oil producers.

These shifts are part of a pattern on the committee, which has broken up into competing factions opposing various elements of a tax package that would spread a significant share of the burden onto the business world.

Recognizing the problems he faces putting together a bill, Dole has repeatedly called on the White House to give strong public support to tax increases, not an easy task for President Reagan, whose conservative posture has been strongly opposed to federal taxes.

The just-passed budget resolution requires Congress to approve in this election year tax increases of $20.9 billion for 1983, and $100 billion over the next three years.

This has touched off intense lobbying on Capitol Hill, as corporations seek to preserve the gains they made in last year's big tax cut.

After passage of the 1981 tax bill, the public recognized that the corporate share of financing the federal government was steadily declining, with profitable companies using the leasing provisions to reduce liabilities to zero. The result was strong pressure to repeal tax leasing and tighten up elsewhere on corporate taxes.

The administration came up with a broad, new corporate minimum tax as a way to counter this public discontent, and Dole called for the complete repeal of the leasing provisions.

Since then, however, aides to GOP senators on the Finance Committee have been meeting regularly. In a privately circulated memorandum dated June 1 that ranks opposition among Republican committee members to various tax proposals, the two increases described as facing "strong objection" were repeal of tax leasing and imposition of the corporate minimum.

Those listed as having "minimal objection," in contrast, would mainly fall on individuals, not corporations. Among the big-ticket items are repeal of the deduction for state sales taxes, yielding $11.8 billion over three years; restrictions on interest deductions, $6.2 billion, and limits on the deductibility of health insurance premiums, $5.1 billion.

In a related development, House Speaker Thomas P. O'Neill Jr. (D-Mass.) conceded yesterday that House Democrats do not have the votes to repeal the third year of the individual tax cut voted last year. "Some Democrats are now saying the only way for the little guy to get any help is with the third year," O'Neill said.