More than a century has passed since President Ulysses Grant asked Congress to propose a constitutional amendment on the line-item veto. Grant got nowhere. President Ronald Reagan asked Congress for the same thing in his State of the Union address the other evening. Reagan won't get anywhere either. This is an idea whose time hasn't come.

Forty-three of our 50 state governors have the power to reach into an appropriations bill and to veto certain individual items while approving all the rest. Only Indiana, Maine, Nevada, New Hampshire, Rhode Island and Vermont have denied their chief executives this authority. (North Carolina's governor has no veto power of any kind.)

If the line-item veto existed at the federal level, Reagan might have delved into the monstrous $604 billion omnibus bill that Congress dumped in his lap just before Christmas. By way of example, the president might have vetoed the $8 million sought by Sen. Daniel Inouye to build a school in Paris for the children of North African Jewish refugees. It is unlikely that, on reconsideration, two-thirds of each house of Congress would have voted to override.

The line-item idea did not originate with Grant. It originated with the Confederate States of America in their constitution of March 1861. President Jefferson Davis was given power ''to approve any appropriation and disapprove any other appropriation in the same bill.'' In 1873 Grant got fed up with the congressional practice of attaching nongermane riders to bills that reached his desk. Grant tried to persuade Congress to give him the line-item veto, but no one was interested. Over the years Presidents Hayes, Arthur, Franklin Roosevelt and Nixon have made the same pitch.

The arguments in favor of such a presidential power have been strengthened by the irresponsible behavior of Congress in the past two years. Instead of sending 13 separate appropriations bills to the White House, as the budgetary process requires, Congress has packed everything into a single indigestible ''continuing resolution.'' Take it or dare to leave it, says Congress. A reluctant Reagan, facing shutdown of the government with Congress in recess, had no choice. He signed. It is a terrible way to run our public affairs.

But a line-item veto is not the right remedy. Under the Constitution, the legislative and executive powers are in a delicate balance. There is a nice tension here. We ought not to disturb the arrangement without compelling reason. Presidents have the full-blown veto power given them by the Constitution, and they have made it work. Since the presidency of George Washington, presidents have vetoed more than 2,400 bills, and all but a hundred vetoes have been sustained.

Presidents have other weapons against a spend-happy Congress. They have considerable discretion in deferring actual expenditures. Presidents may rescind particular items, subject to congressional approval within 45 days. They have the power of the bully pulpit to rouse public opinion. The threat of a veto usually has a profound effect.

There is this objection also -- that even the most vigorous wielding of a line-item veto would not significantly affect the federal deficit. The power would not apply to such major outlays as interest on the debt, Social Security benefits, Medicare, Medicaid and other entitlement programs. When all the untouchables have been accounted for, not much that is touchable remains.

A further difficulty is that there is no political constituency for the constitutional amendment the president repeatedly has requested. Various organizations are supporting a quite separate amendment to compel a balanced federal budget, but no one is really hot for the line-item veto. Sen. Bob Dole, with the cosponsorship of 19 colleagues, has introduced a line-item resolution in the upper chamber. Seven similar resolutions are pending in the House. No hearings are in prospect.

The line item works at the state level, as Reagan is fond of recalling. During his eight years as governor of California, he invoked his power 943 times and never was overridden. But it works mainly as a device for enforcing the balanced budgets that are required by state constitutions.

At the heart of the political process lies the delegation, the exercise and the restraint of power. We have lived 200 years with the present system by which powers are divided more or less precisely among the three branches. To grant the line-item veto would tilt the balance by giving presidents more power than presidents ought to have.