IT'S in the Contract With America. The House has passed a version of it. Most Senate Republicans, and a lot of Democrats, want it. President Clinton is begging to have it.
So why is passage of a ''line-item veto'' so much in doubt in the Senate? Perhaps because, with Mr. Clinton likely to sign just about any version that reaches his desk, senators want to get the details right.
Congress holds dear its power of the purse; in giving the president more ability to cut pork-barrel spending, it doesn't want to give away the store.
For a time, it looked as though the Senate would choose between two competing Republican bills that would give the president more power to make ''rescissions,'' that is, to send portions of legislation back to Congress for reconsideration. Legislation from Sen. John McCain (R) of Arizona most closely matches a bill passed by the House Feb. 6. This ''enhanced'' rescission would require a two-thirds vote to override any spending cuts made by the president.
But many senators worry that this would give too much power to the president. A bill from Sen. Pete Domenici (R) of New Mexico would have granted ''expedited'' rescission and required only a majority vote to override the president. Senator Domenici argued that his bill puts enough of a spotlight on pieces of pork to keep senators from overriding the president too easily. His measure found support among some Democrats, but lost the conservative Republicans, who saw it as too weak.
Now Republicans are exploring yet another approach called ''separate enrollment.'' In it, appropriations bills would be ''atomized'' into separate bills, one for each item of spending, after they were passed by Congress but before they go to the president. The president could then rescind any of the individual parts.
Democrats are expected to offer yet another rescission plan this week.
While any version would likely be of some help, the most useful bill would expand rescission power beyond appropriations to include tax provisions and new entitlements. It would have a ''lockbox'' feature, requiring the money saved to be applied to reducing the deficit.
And it would allow the president not only to eliminate entire items but also to trim them back, if he chooses.
Giving the president more power to rescind isn't going to solve the deficit problem, but every little bit helps.