If you're one of thousands of American homeowners with a home-equity line of credit, equity loan or second mortgage, keep your eyes on two little-noticed proposals from Congress's top tax-law specialists.

New recommendations from the influential Joint Committee on Taxation could sharply curtail the deductibility of the interest you pay on your mortgage debt -- perhaps as early as next year.

The committee, though it has a relatively low profile among the general public, serves as Congress's in-house source of technical expertise on all tax matters. Tax legislation commonly is drafted by the committee's attorneys and economists. When members or staff on either the House or Senate tax-writing committees need guidance on tax-policy issues, they turn first to the nonpartisan joint committee.

The pivotal role played by the committee makes its two new proposals particularly significant for homeowners. The recommendations are part of a larger package of suggestions on how to simplify the federal tax code. The main objective, according to the committee, is to eliminate complexities relating to interest deductions.

One proposal would eliminate distinctions among home-residence, investment and personal interest. Before the Tax Reform Act of 1986, interest on personal debt for such expenses as automobiles or college tuition was deductible.

The 1986 law, however, began a phase-out of those deductions. In tax year 1990, 10 percent of interest on personal, nonbusiness debts can be written off. The 1986 law left only one large type of personal debt eligible for interest deductions: "qualified home-residence" interest, chiefly first and second mortgages or equity lines tied to your home. Interest on home-equity loan debt is fully deductible -- no matter how the money is spent -- as long as the loan does not exceed $100,000.

Because of the 1986 changes, many homeowners have taken out equity lines of credit or conventional second mortgages. Since the interest is deductible, they have used their home equity to finance cars, vacations, home furnishings and similar expenditures.

The joint committee, in its new proposal, would remove what it calls "the incentive for individuals to structure loans as home-equity debt in order to avoid the present-law limit on the deductibility of personal interest."

It would achieve this by putting a 70 percent to 75 percent limit on deductions for all nonbusiness interest. Lumped into one category for tax purposes would be consumer loan interest, revolving charge accounts, unsecured personal loans, student loans, investment debt and home mortgage debt on first and second homes. You'd add up all your interest at the end of the year, and then multiply by 0.70 or 0.75 to get your limit. The 25 percent to 30 percent of interest left over would not be deductible, and could not be carried forward to later years.

What would such a change do to you? It would depend on how much mortgage interest you now deduct. If you live in a high housing-cost urban area and took out a home-equity loan for the reasons cited by the joint committee, the new plan would leave a sizable dent in your personal economics.

If, on the other hand, you have a modest-size home mortgage and relatively large personal debts that are not deductible, you could be a beneficiary at tax time.

The second proposal also would require taxpayers to add up all nonbusiness interest paid during the year, including home mortgages. Then you'd add up the amount of your "net investment income" for the year -- such as stock dividends and bank account interest. You could then write off as much nonbusiness interest as you have net investment income. The proposal implicitly recognizes that many homeowners do not receive significant amounts of investment income, and therefore suggests the cap be raised by some unspecified "additional dollar amount."

Depending upon what this amount might be, the proposal could dig far deeper into your current mortgage deduction than the first one.

Several top congressional tax experts said one of the proposals could be pushed into the tax bill now taking shape on Capitol Hill. "Anytime a concept like this originates in Joint Tax," said one staff member, "you've got to take it seriously. Once they're on paper, these things tend to jump out of the drawer {into legislation} when everybody's looking somewhere else."

Another staff member said he would be "shocked" if the forthcoming Bush-Democratic tax summit agreement included any such proposal. But, he added, it could "easily" surface later this summer when the joint committee staff drafts the 1990 tax bill language.