Nearly every weekend, Sen. George Mitchell (D) returns to his home state of Maine and holds public meetings. In town halls from Portland to Caribou, constituents stand up and talk about what's on their minds.

And with regularity, Mitchell says, someone asks: " 'Will there be a tax increase? What kind of taxes will be increased?' "

Taxpayers know, in other words, what politicians are afraid to admit: Barely a year after Congress enacted the most sweeping overhaul of the tax code in history, and sponsors swore they would leave the system alone for a while, tax increases are lurking.

Even before the first returns have been filed under the Tax Reform Act of 1986, the House has passed legislation that would raise $12 billion in new taxes in 1988, while the Senate Finance Committee has approved an $11.6 billion package.

To add to the confusion, congressional leaders are meeting with representatives of the Reagan administration to fashion a deficit-reduction package that may or may not include the tax increases already approved in the House and Senate. At week's end, they were still far from agreement on a tax-increase and spending-cut package.

Some of the provisions in the House and Senate bills (which have about $6 billion worth of revenue increases in common) would alter the brand-new 1986 tax code. Of those changes, some would crack down further and some would provide relief from the law's strictures.

Still unclear, of course, is exactly what will emerge when the House and Senate sit down to reconcile their differing bills -- assuming the full Senate produces a tax bill and President Reagan does not veto the final product.

The uncertainty clouds even the simplest of financial-planning strategies. For instance, advisers routinely recommend that taxpayers postpone receiving income until the following tax year if possible. That strategy makes even more sense for next year, when the top individual income-tax rate is scheduled to drop from 1987's 38.5 percent to 28 percent, or 33 percent at certain income levels.

But one of the provisions in the Senate Finance Committee tax bill would apply the Medicare payroll tax of 1.45 percent to all income, rather than only to income below a certain threshold -- $45,000 next year. So what now appears to be a 33 percent top rate for 1988 could well be a 34.5 percent top rate -- enough, perhaps, to make it worthwhile to take the money now.

"Just a couple of percentage points can make it all for naught," said Stephen R. Corrick of the accounting firm Arthur Andersen & Co. Corrick suggested making financial plans based on sound economic sense, rather than placing too much emphasis on tax considerations.

Raymond E. Lang, head of a Bethesda accounting firm, said the confusion is upsetting to his clients -- and to him.

"We are sort of despairing of the present tax situation," Lang said. "We feel a halt has got to be called to making changes unless it is to correct the tax reform act of 1986, simply because we really question whether we can comfortably advise clients about what to do."

Numerous provisions in the House and Senate bills could affect consumers' financial plans by their enactment -- or nonenactment. Among them:

The House bill would tax each year a portion of the difference between the price of bonds bought at a discount and their face value. Now, investors who buy bonds in the secondary market at less than par value pay taxes on the "gains" when the bonds mature; the proposal would tax the gains on a pro rata basis each year. The provision is not in the Senate bill, but the recent disclosure of the proposed change caused considerable consternation in the bond markets last week. Both bills would delay for a year the 1986 law's tightening of estimated-tax payments. Starting this year, taxpayers who use the estimated basis must pay 90 percent of their ultimate bill in quarterly payments during the year (unless they satisfied the alternative requirement of paying 100 percent of the previous year's liability). The pending bills would set the payment threshold at its former level of 80 percent, just for 1987. Both bills would repeal a 1986 provision sure to anger investors in stock-market mutual funds. Under last year's law, investors would have to pay tax on a portion of the mutual funds' operating expenses as if it were income to them, even though they never saw the money. The pending bills would do away with what has been dubbed the "phantom income" provision. The House bill would alter a popular way to escape the new law's phase-out of the deduction for nonmortgage interest (65 percent of such interest is deductible this year; 40 percent in 1988). To evade the limits, consumers increasingly have been taking out loans using their homes as collateral, deducting the interest as mortgage interest.

The 1986 law limited the deduction for home-equity loans to interest on loans equal to the original purchase price plus the cost of improvements, with an exception permitting unlimited deductions if the money was used for educational or medical expenses.

The House bill would change that home equity loan limit to an absolute one permitting deductions on loans against a home in amounts of $100,000 or less, up to the fair market value of the residence.

And it would restrict the deduction for interest on mortgages on first and second homes to a total of just over $1 million.

Experts say some taxpayers would be better off under current law if, for instance, they paid more than $100,000 for their houses or wanted to use the money for tuition or medical costs. Others, such as those who bought their homes some time ago, could take larger deductible home-equity loans under the House proposal.