If you care about home mortgage interest deductibility, keep your eye on both the House and Senate tax-writing committees in the next 30 days. Despite early signs that partisan politics may block tax legislation this year, reformers on Capitol Hill are still gunning for home-equity loans and other real estate targets.

As long as tax-revision sessions continue -- and they're currently under way or on the docket in both houses -- the write-offs you assumed were absolutely safe are not. Not until Thanksgiving.

Here's a quick update on what's happening, and why:

From early October until mid-November is revenue-raising season this year on Capitol Hill. Under the Gramm-Rudman-Hollings deficit-control legislation, Congress will need to come up with $12 billion in new tax revenues for fiscal 1988 -- and fast.

The whole package will have to be enacted by Nov. 20 or deep domestic- and military-spending cutbacks will go into effect automatically. House and Senate tax committee members hunger for money. Normally, even such revenue appetites wouldn't cause them to target one of the most politically secure sections of the tax code: mortgage interest deductions on first and second homes. No one in his right mind would consider revisiting an issue set in concrete as recently as last year, right?

Wrong, according to key experts working with the House and Senate tax-writing committees. There is significant support -- particularly among the committee staff members who design, recommend and actually write legislation -- for revisions this fall to last year's home-equity interest deduction tax solutions.

In the words of Senate Finance Committee Republican staff member Greg Jenner, their goal is to "tighten down" on the 1986 formula that allows Americans to write off "consumer interest" expenses by effectively tacking them onto their home mortgage debt.

Key members of the Senate, according to Jenner, are appalled at 1987's home-equity loan boom. While the 1986 tax law pared away traditional write-offs for consumer items -- auto loans, charge cards and the like -- it created an easy way for taxpayers to thumb their noses at that policy. By refinancing first mortgages or by taking out some form of second loan, homeowners can borrow tax-deductible dollars -- using credit cards in some cases -- far beyond the original mortgage on their house.

Jenner told a real estate tax law conference last week that the home-equity borrowing binge deeply worries some members of Congress. Down the road, in tougher economic times, heavily leveraged homeowners could default on their real estate loans, lose their property and shake the nation's financial system to its foundation.

This is precisely why sentiment is growing to rein in mortgage deductibility, according to one of Jenner's counterparts at the House Ways and Means Committee. James Clark, minority tax counsel on the panel, told the same conference that there is a "likelihood that modifications will be made" if House members focus on home-equity loans.

Among the types of changes he said are possible this fall are new limits on future mortgage deductions to no more than acquisition indebtedness. That means if the mortgage on your $100,000 house was $80,000 at purchase, you couldn't deduct interest on any larger loan secured by that house in future years, whether through refinancing or equity loans. No cash out, tax-free refinancings of inflated equity. No lines of credit.

Clark also suggested that tightening up on second-home deductions secured by pleasure-boat loans would find support on the committee. "I suspect something might be done along those lines," he said.

National tax policy would not be Congress' sole or even primary motivation for tax switches such as these. David Brockway, staff director of the influential Joint Congressional Committee on Taxation, noted that although "you can make strong policy arguments against {current tax treatment of} home-equity interest deductibility," the revenue-raising potential of such changes could be even more compelling to tax writers.

Brockway's staff estimates that by limiting home-equity interest deductions to "acquisition indebtedness," nearly $4 billion in federal revenues could be gained between now and fiscal 1990. That's a big number, even on Capitol Hill