ONE OF RONALD Reagan's cherished goals for his second term is to get from Congress the authority to veto specific items of government spending in appropriations bills -- the so-called line-item veto. In televised addresses, budget messages and stump speeches, Reagan has called for this power, saying that he will make the tough decisions that the gutless, log-rolling lawmakers won't.

The president came close to achieving a two-year trial for the idea in the Senate before it recessed, falling only two votes short of breaking a filibuster. His chief ally, Sen. Mack Mattingly (R-Ga.) has vowed to bring it up again and again until he prevails.

Lost in all the furor of the line- item veto debate is a fundamental and curious fact: The president already has the equivalent of a line- item veto. This president knows it; he used it to block nearly $15 billion in spending in his first year in office. But since then, Reagan has evidently decided that he doesn't much like the weapon.

Last year, even as his outrage over burgeoning deficits reached a crescendo, his use of this potent tool dropped to almost nothing. He excised little more than one-half of one billion dollars, or less than 1/400th of the deficit.

What is the line-item veto authority already in the hands of the White House? It's called the recission, and it was a key component of the Budget Act of 1974. Back then, Congress wanted to limit, but not eliminate, the president's power of impoundment that Richard Nixon had abused, provoking a court case and ultimately that same budget act. In that act, Congress created two classes of presidential authority to block appropriated funds: recissions and deferrals.

Deferrals -- areas where the president wants to defer spending until the following year because the spending cannot be absorbed -- are announced by the president and approved unless a majority of both houses of Congress specifically votes to disallow them.

Recissions don't defer spending -- they block it altogether. The president has the power to rescind spending at every level, to the point of eliminating specific projects or parts of projects rather than categories -- a degree of specificity far greater than the line-item veto authority could ever grant him.

It is true that for the recission to take effect -- to actually block the spending -- a majority of both houses of Congress has to pass a resolution approving the president's action within 45 days. But in 1981, Congress approved 89.5 percent of the $16.2 billion in recissions that Reagan proposed. Thus, even though the recission power is weaker than an outright veto (which requires two-thirds of both houses of Congress to override it), Reagan himself has shown that it can work remarkably effectively.

With this potent weapon already available, why has Reagan stopped using it? And why does he spend so much time and energy demanding new authority that is in so many ways redundant? It is possible that, with his penchant for glossing over the details of government, he has simply forgotten about it. But surely David Stockman, the wizard of budget arcana, brought it to his attention many times.

It is possible that he thinks the power is too weak, and that he would simply be overridden by Congress if he attempted large-scale use of the recission line-item veto. But past experience suggests that he would win his share of these battles.

In any case, if the president's reluctance to use the recission power is based on its weakness, why doesn't he make a major effort to strengthen it? A simple legislative change in the 1974 act could make recissions the same as deferrals, putting the onus on both houses of Congress to act if they insist on spending for the items that the president wants to block.

It would even be possible to make these actions subject to a presidential veto, which would require a two- thirds vote to override. This kind of straightforward change in statutory authority is clearly preferable to an act of dubious constitutionality like the line-item veto bill the president now supports.

But in fact, the weakness of the current recission power is not the issue. If the president simply used his current power to item-veto tens of billions of dollars in spending, taking on popular programs and special interests he insists he is willing to take on, he would put the heat on Congress. If Congress spurned him, it would dramatize emphatically where the real responsibility for deficits belongs.

We may get the specific line-item veto authority the president is so adamantly after sometime later this year. In the meantime, we have a real and tangible test of the president's sincerity. Let him use the line-item veto he already has.