ONCE PRESIDENT Reagan and the congressional Democrats decide to agree on the budget, hitting the specific targets won't be exceptionally difficult. The chairman of the House Ways and Means Committee, Dan Rostenkowski, made that point last week when he circulated a list of possible tax increases. Some were drawn from Mr. Reagan's budget last January, and none would make much of a ripple in the economy. But the budget is currently caught in the usual struggle over symbolism and Mr. Reagan's much-repeated pledges never -- no, never -- to allow any sort of a tax increase.

Congress has now put itself under a requirement to cut the deficit by $23 billion in the fiscal year that began last Thursday. Half the $23 billion is to come from reductions in spending and half from higher revenues. Where might those revenues be found? Mr. Rostenkowski offered $8 billion worth of suggestions. How about extending Medicare coverage -- and payroll taxes -- to all state and local government employees? How about increasing the excise tax on coal to pay for a larger share of black-lung benefits for miners? How about charging user fees for private ruling letters from the IRS? All of those were proposals that Mr. Reagan made at the beginning of the year. The telephone excise tax is about to expire. Why not extend it? And so forth. Under present law the federal tax system will collect $900 billion this year. Adjusting it to pick up another $12.5 billion is not impossible.

Not impossible, that is, unless Mr. Reagan is determined to take a more inflexible stand against new taxes than he did in January. If he does, the Gramm-Rudman-Hollings rules will begin imposing automatic spending cuts. These cuts would take effect on a tentative and reversible basis Oct. 20. On Nov. 20, they would become final and irreversible.

Automatic cuts across the board are never a good way to manage a budget. The virtue of the G-R-H rules is that by promising an alternative that is harsh and damaging they give people an incentive to compromise rather than to posture.

That Nov. 20 deadline is artificial, invented by Congress for the purpose of forcing decisions. But it is real. Congress and Mr. Reagan have given themselves a little over six weeks to come to terms with a modest and not very painful tax increase