Arguments about the line-item veto boil down to the distinction between that which is persuasive and that which is convincing. Sen. Paul Simon (D-Ill.), the chief advocate of this executive device, is persuasive. In the end his arguments are not convincing. This is a bum idea that Congress should abandon.

The senator's case is to this effect: Congress in recent years has adopted a parliamentary tactic that is unfair to a president and bad for the country. The trick is to wrap hundreds of individual appropriations into one bulging package and to drop the thing on a president's desk. A message is attached: take it or leave it.

Many of the items in such an omnibus bill never would survive a straight up-or-down vote on the floor. They are the stuff with which pork barrels are filled. Often these frivolous or extraneous items sneak into a bill without even a moment's hearing in committee.

To remedy the situation, Simon proposes an amendment to the Constitution: ''The president may reduce or disapprove any item of appropriation in any act or joint resolution, except any item of appropriation for the legislative branch of the government.'' By a majority vote in each house, the vetoed item could be restored. Simon's resolution has cleared committee and is headed for the floor.

Simon notes that 43 states accord substantially this power to their governors. As governor of California, Ronald Reagan used his line-item veto 943 times. He was never overridden. During the past seven years, Illinois governors have reduced or eliminated items that have saved $2 billion. Former presidents Nixon, Ford, Carter and Reagan support the amendment. President Bush has asked repeatedly for it.

The Senate Judiciary Committee conducted hearings on Simon's amendment on April 11. There the opposition made a convincing case against it. Orrin Hatch of Utah summed up the principal argument in a single sentence: ''I think it would result in a massive shift of power from the legislative to the executive branch.''

Exactly so. Our Constitution rests upon two solid foundation stones. The first is federalism, which is not involved here. The second is the separation of powers among the executive, legislative and judicial branches. For 200 years these powers have rested in a nice equilibrium. The system hasn't been perfect -- the judiciary often has assumed disturbing power -- but it has worked.

The Simon amendment would upset the balance in critical ways. Given the power of line-item veto, a president would be in a dominant position to wheel and deal, to trade for votes on his own projects, to practice a kind of genteel extortion. Let us suppose that a president is truly determined to have his way on a manned station in space. The Senate is cool; the House is barely lukewarm. Along comes a massive bill to provide supplementary appropriations. It contains a hundred pet projects beloved by their sponsors.

Well, says the president, if you gentlemen will give me my meritorious space station, I won't veto your unwarranted dams. A deal is struck. The cats and dogs stay in the bill. Nothing useful has been accomplished. And the line-item veto has functioned not as a scalpel, but as a billy club.

Wholly apart from objections based upon principle or pragmatism is a technical third point raised by Sen. Mark Hatfield (R-Ore.). He says the line-item veto, as a practical matter, isn't what it's cracked up to be. It simply would not accomplish what Simon believes it will accomplish.

Sixty percent of the budget, representing entitlement programs and interest on the debt, couldn't be reached by a line-item veto. Three-fourths of the remainder is military spending. That doesn't leave much.

Moreover, most appropriations are by generalized ''accounts,'' such as a military construction account. Individual items within an account may be described in a committee report, but they do not appear in the bill itself.

The fact that 43 governors have a line-item veto is interesting but irrelevant. States can spend money, but they cannot print money. Their powers of appropriation thus have built-in limitations.

If it is not necessary to amend the Constitution, it is necessary not to amend the Constitution. No compelling case has been made for a dangerous transfer of enormous political power to future presidents. The Senate should kill the resolution.