Prospects for final action this year on President Reagan's package of tax hikes appear virtually nil as both Democrats and Republicans are considering major additions and alterations of the administration's proposal.

The tax hikes, described elliptically by the administration as tax "enhancement" proposals, are key elements in the drive to lower the deficit in 1982 by $16 billion and integral to the long-range drive to balance the budget by 1984.

The administration does not plan to have its legislation ready until at least Oct. 20, and probably later. Congress now plans to try to adjourn by Nov. 20, and even if it stays in session until Christmas, it is extremely unlikely, according to near unanimous assessment by Democrats and Republicans, that final action on a controverial tax package could be completed by then.

Senate and Congress are considering significant cutbacks in one of the most controversial provisions called leasing. Under this provision, companies with little or no profits will be able to sell tax breaks they cannot use to profitable companies, in effect creating a market in tax shelters.

Sources on the Senate GOP side said Senator Robert Dole (R-Kan.), chairman of the Senate Finance Committee, is considering amendments that would restore some of the restrictions on leasing so that the contracts would be more than the paper transactions permitted under the new law only to provide a legal vehicle for the sale of corporate tax breaks.

Among possible amendments would be one raising the minimum investment required by the lessor from 10 percent -- an amount that can be regained immediately by cashing in the 10 percent investment tax creditk -- back to 20 percent or higher.

House Democratic aides said members of the Ways and Means Committee are taking a similar look at the leasing section, which is conservatively estimated to cost the government $27 billion through 1986. They said Democratic members consider the provision fair game because it was never included in the Democratic version of the tax bill that was offered as a substitute for the Reagan package last summer.

In fact, however, the leasing provision may find significant support among the Democrats because it gives the largest benefits to industrial companies in the Frost Belt, an area where the party's voting strength is perhaps strongest.

In a more radical alteration of the administration's revenue-raising package, House Democrats are considering restoration of many of the oil taxes that were cut in the Reagan bill. During consideration of the earlier measure, the Democrats had approved a package of reductions in the windfall-profits tax, but the action was taken largely as part of the bidding war for votes, and many Democrats voted for the oil cuts not because they believed in them, but in a futile effort to pick up support from oil-state Democrats.

In addition to providing an alternative to the tax hikes the Reagan administration has tentatively outlined, all of which face strong opposition from entrenched lobbies, withdrawing the breaks for the oil industry would be a way to politically embarass Rep. Kent Hance (D-Tex.). Hance was the only Democratic member of the Ways and Means Committee to defect to the GOP in the tax fight, and he was a princple cosponsor of the Republican measure.

Another proposal being explored in the Senate Finance Committee is reducing or eliminating the deduction for interest payments except for home mortgages and car loans. However, legislation along these lines would run into intense opposition from banks, finance companies and a host of other groups involved in consumer lending.

In announcing plans to lower the deficit by $16 billion, the Reagan administration called for $13 billion in budget cuts in addition to those provided in the $35 billion reconciliation bill and for tax "enhancement" measures that would raise a total of at least $3 billion in 1982.

The package of tax increases include proposals that consistently have run into trouble on Capitol Hill. Among them are requiring government contractors--primarily defense contractors--to pay taxes as they receive payments instead of postponing tax liability until the completion of the contract, eliminating a provision in the law that allows insurance companies to cut tax liabilities ion half when they share these liabilities with a subsidiary, and lowering the exemption ceiling on unemployment compensation payments from the current level of $20,000 in total income for a single return and $25,000 on a joint return.

Meanwhile, in remarks prepared for delivery in Los Angeles Treasury Secretary Donald Regan said last night that the administration will not consider watering down any of the tax cuts already passed by Congress and still is committed to $16 billion in budget cuts for 1982.