A Senate Democratic task force is expected to consider a series of options today for possible tax cuts early next year, including Social Security tax relief for individuals and faster depreciation writeoffs for businesses.

The proposals, outlined in a memorandum by Sen. Russell B. Long (D-La.), chairman of the Senate Finance Committee, are designed to help hammer out a consensus plan on the taxcut question by Sept. 15.

The group, led by Sen. Lloyd Bentsen (D-Tex.), chairman of the congressional Joint Economic Committee, is scheduled to meet late today to review the Long proposals and consider recommendations on related issues.

However, sources said it still isn't clear whether the group could reach agreement by the Sept. 15 date. Senate Democrats earlier had pledged to report a tax bill by Sept. 3 but this since has been postponed.

Congressional aides cautioned that Long's list is only a menu of alternatives that the Senate Finance chairman believes could be considered and does not amount to a set of recommendations for legislation.

The Democrats are struggling to respond politically to Republican presidential candidate Ronald Reagan's call last month for a 10 percent across-the-board cut in individual income taxes and faster depreciation writeoffs for business.

Both the House and the Carter adminsitration are trying to stave off congressional action on a tax-cut bill anytime this session in hopes of pushing through their own legislation early in 1981.

Among the options Long listed for tax cuts for individuals:

Averting the sharp Social Security tax increase now scheduled for Jan. 1 by either freezing the Social Security tax rate -- and using general revenues to make up the loss -- or by allowing an offsetting credit on income taxes.

A general income-tax cut of between $14 billion and $16 billion in 1981, aimed primarily at low-and middle-income taxpayers. If Congress wants, Long said, the cut could be targeted to offset the payroll tax hike.

A move to eliminate the so-called "marriage penatly" -- the quirk in existing law that taxes two-earner couples more heavily than if they were couples more heavily than if they were single -- by allowing marrieds to file separate returns or providing them with a special deduction.

Miscellaneous tax breaks for charities, such as allowing taxpayers who do not itemize to claim deductions for contributions in addition to the standard deduction. This proposal, rejected by Congress last year, would cost the Treasury $3.6 billion.

The proposals involving business include several options for providing faster depreciation writeoffs, an expansion of the investment tax credit, a further cut in corporate tax rates and a new cut in capital gains taxes.

Long also raised the possibility of enacting new incentives for savings from broadening the recently enacted exclusion for interest and dividends to liberalizing the tax treatment in the case of dividends that are reinvested.

Besides the question of possible tax cut proposals, the task force also is scheduled to consider position papers on plans for revitalizing the ailing housing industry, money and credit policies and labor and other issues.