Should President Reagan be given the power he is seeking to veto individual items in spending bills passed by Congress? Today, he must veto, or threaten to veto, an entire appropriation bill to get at specific provisions he finds objectionable. Would an item veto give him a more effective weapon to reduce the deficit?

As a believer in the federal system, I was strongly inclined, by the example of 43 states that give their governors the item-veto power, to think the president should have similar authority. But as I began studying the issue, arguments from fiscal and constitutional conservatives made me understand this is not the simple proposition it seems.

The Senate opponents of the measure include two southern Democrats, Howell Heflin of Alabama and John Stennis of Mississippi, who served as judges in their states and are as consistent in their conservative interpretations of the Constitution as any two men in Congress. When Stennis says, as he did in debate last week, that giving this power to the president "would gradually destroy the legislative branch of this government," it is not prudent to disregard the argument.

The same thing is true about one of the most vocal opponents of the companion House measure, Rep. Mickey Edwards (R- Okla.). Edwards, a former law professor, has headed the American Conservative Union and is a longtime supporter of Reagan's. The last thing in the world he wants to do is thwart Ronald Reagan, but he has been conducting a furious correspondence and lobbying campaign with fellow-Republicans to persuade them that the line-item veto is a real Trojan horse.

Of course, presidents already have the power to veto appropriation bills, as they do any other form of legislation. But they must veto the whole bill and risk being overridden if two-thirds of the senators and representatives are persuaded that the bill, on balance, is a good one.

But if the president can single out an individual item in the comprehensive appropriation bill for veto, he is taking on only those legislators with a special interest in that item.

In Edwards' view, that "shifts the president's power from the level of persuasion to the level of blackmail." One of Edwards' arguments is that a future Democratic President -- a Gary Hart, say -- could use an item-veto to kill the MX missile or B-1 bomber program and could make the veto stick even if all 435 House members and 64 of the 100 senators thought that decision risked the nation's security.

For those who don't see great likelihood of a liberal Democrat's succeeding Reagan in the White House, Edwards appeals to the traditional legislative sense of power and prerogative.

And here, he and the more liberal Republicans sch as Mark Hatfield of Oregon and Charles McC. Mathias of Maryland, who are leading the Senate opposition to Reagan's request, are certainly on sound political and constitutional ground.

Mathias provided an example in debate of what the shift might mean in his case. "For example," he said, "if President Reagan does not like my position on the issue of school prayer, and if he acquires the power to kill funds for the program that I have long supported to save the Chesapeake Bay . . . then the president . . . has a hostage.

"He can hold the Chesapeake for the ransom of my support . . . for state-sponsored prayer in school or any other subject that he might want my support on. . . . In my opinion it would destroy the balance that exists between . . . the executive and legislative branches."

That it would shift power is unquestionably correct. Whether the critics are right in saying it would "destroy the balance" and cripple the legislative branch is considerably less clear.

Oklahoma's governor has the item-veto power, backed by a requirement for a balanced state budget, and yet Edwards concedes that its Legislature is hardly a toothless, enfeebled institution.

When Hatfield, a former governor of Oregon, was expounding on the "mischief" a presidential item-veto could bring, his Republican colleague Dan Evans, a former governor of neighboring Washington, said it hardly seemed "radical" to him for the national government to do what 43 states already do.

It is a close question, in my view. Experience might show that the item-veto power is no more malign in the hands of a president than it has been for governors. But these are not ordinary times. We have a budget deficit crisis and any president given this new power would be tempted to use it, as Hatfield said, for "radical" surgery. Much as I believe that the deficit needs to be cut, I am not convinced that it would be wise policy or healthy precedent for Reagan to be given a greater degree of unilateral power to determine how the cutting is done.

On balance, I think it best that he be forced to negotiate those decisions with Congress -- and Congress with him -- as the makers of the Constitution intended. On this issue, I think the conservatives are right, and Reagan is not voicing the conservative position.