IN the 200 years since the first 10 Amendments were ratified in 1791, the Constitution has been amended 16 times. An earlier amendment was repealed, making the net total 14. President Bush wants to add five more, a strange effort by a conservative president to fix so many things that aren't broke.Mr. Bush wants to require a balanced budget, to give his office a line-item veto in appropriation bills, to outlaw burning the flag, to permit prayer in public schools, and to outlaw abortion. Congress, to its credit, has rejected the flag-burning amendment as well as the prayer in schools amendment. The balanced budget amendment is fiscal fantasy. If the Supreme Court eventually reverses itself on abortion, that issue will be played out in 50 state capitals, to the relief of many people in Washington. The line-item veto is of more immediate concern. This gives the president authority to veto individual items in appropriation bills without vetoing the whole bill. Congress has rightly resisted such a massive shift of power from the legislative to the executive branch. But Bush is exercising what amounts to a line-item veto by disapproving entire bills because of single items. And Congress is letting him get away with it. Three times Bush has vetoed appropriation bills for the District of Columbia because they allowed the district to use its own money (not federal money) to pay for abortions for poor women. Twice Congress has dutifully re-passed the bill without the offending provision, and Congress may do so again later this month. Because of disagreements over single provisions, Bush has vetoed appropriations bills for foreign aid, for the Departments of Labor, Education, and for Health and Human Services (HHS). He has vetoed appropriations bills for the intelligence community and the State Department. Today even the threat of a veto scares Congress. Under White House threats, Congress has abandoned or diluted provisions in foreign aid appropriation bills, in authorizations for the National Institutes of Health, and HHS. A similar threat on a gasoline tax in a highway bill is pending. Any president is going to use all the power that Congress and the courts let him get away with. Franklin Pierce and Calvin Coolidge were exceptions. George Bush is not. There is nothing new about a conflict between the president and Congress over what appropriations can be used for, and over where taxes come from. A quick review shows why: IT used to be that no president would dream of vetoing an appropriation bill. Either the veto would be overridden or the activities funded by the bill would cease, a consequence too horrible to contemplate when it affected a major department of the government. But then, presidents had other remedies. During the Truman administration, Congress earmarked funds for Franco's Spain, and President Truman refused to let them be spent. He took the view - and made it stick - that although Congress could appropria te the money, Congress could not force him to spend it. This, of course, amounted to a line-item veto. Twenty years later, President Nixon carried this to such extremes, for domestic programs, that Congress prohibited it without prior notice and congressional acquiescence. In effect, this took away the de facto line-item veto represented by impoundment and confronted the president with the choice of either congressional action or responsibility for a major government disruption. Another 20 years later, Bush confounded Congress by vetoing appropriation bills and transferring to Congress the responsibility for disrupting government. If Congress wants to reclaim the power it took from Nixon, it is going to have to stop letting Bush get away with this. If two-thirds of Congress is unwilling to override an appropriation veto, then a majority of Congress has to be willing to let the whole bill die and face the consequences. It has to send back to the president time after time the same bill he vetoed and make the president responsible for the ensuing chaos. Until Congress does this, Bush (and his successors) will keep on using a de facto line-item veto. And in so doing, the president will undermine his own case for amending the Constitution, because he will have already acquired the power he seeks. It would be hard to find a better example of constitutional flexibility or a better reason for letting nature take its course in the ebb and flow of power between president and Congress.