WASHINGTON (AP) _ The Senate this week turns its attention to greatly expanding the ''Superfund'' program as Congress races to meet an Oct. 1 deadline for renewing the nation's most ambitious toxic waste cleanup effort.

Senate leaders said Monday that a $7.5 billion Superfund bill will be debated, starting either Tuesday or Wednesday, after the chamber completes work on legislation to revise the nation's immigration laws.

Environment Committee Chairman Robert T. Stafford, R-Vt., said he expects upwards of 40 amendments to be offered to the Superfund bill but doesn't anticipate delaying tactics by members opposing its price tag and taxing language.

The Senate bill, proposing a nearly five-fold increase over the $1.6 billion budgeted for Superfund's first five years, has been ready for floor action for about three months.

It has been held up, however, by other major legislation, the August recess, and opposition to its main tax provision from the Reagan administration and Senate Majority Leader Robert Dole, R-Kan.

The provision would create a new value-added tax on large manufacturers to raise an estimated $5.4 billion for the program through 1990. Superfund is now financed largely through a special levy on petroleum and basic petrochemicals.

Even through the Treasury Department, Dole and manufacturing groups have complained about the value-added tax proposed by the Senate Finance Committee, no one has offered a concrete alternative for raising the money to pay for an expanded toxic cleanup effort.

Leaders on both sides of Congress are pessimistic about getting a bill to President Reagan's desk before the existing law expires at midnight Sept. 30. A top-level House strategy meeting last week discussed the possibility of buying some extra time through stop-gap legislation extending the current law for 45 days.

A House bill setting Superfund spending at $10 billion through 1990 appears to be several weeks away from floor action. The measure still has to be cleared by two major committees, Public Works and Transportation and Ways and Means, which now is wrestling with the larger issue of income tax reform.

Anticipating the delays on Capitol Hill, the Environmental Protection Agency last month ordered a virtual halt to cleanup operations at 57 of the nation's worst toxic dumps.

Stafford said he expects much of the Senate debate to center on a package of amendments drafted by Sens. Pete V. Domenici, R-N.M., and Alan K. Simpson, R-Wyo., to limit the financial liability of toxic companies.

Simpson said Monday that with the fast-approaching deadline, he would offer his amendments, accept the votes and make no effort to tie up the bill.

''I think anybody filibustering would do it at their own peril,'' he said. ''If we don't meet the deadline, we'll have to extend Superfund for 45 days.''

The questions of the spending level and the taxes to finance it could be settled separately from the Superfund legislation, according to Sam Richardson, a spokesman for Finance Committee Chairman Robert Packwood, R-Ore.

He said Packwood will try to attach the value-added tax language and the $7.5 billion level to the budget reconciliation bill or a continuing spending resolution, which could be needed to keep the government operating if the reconciliation bill is not enacted by Oct. 1.