Chatting on Constitution Avenue outside the Russell Senate Office Building with the Treasury's top tax expert, Sen. Russell B. Long Set forth a homely analogy to save President Carter and his policy advisers from themselves.

The topic of conversation between Long and Donald Lubick, assistant secretary for tax policy, was capital gains. But here of all things was the senator talking about duck hunting. Aim directly at the duck flying by, and you're sure to miss him, said Long; aim a little ahead of the duck, and you have a chance to hit him.

The meaning of Long's analogy: If you measure the tax yield of a reduced capital-gains-tax rate by simply applying the lower rate to existing gains, you are sure to miss the mark; but if you estimate the larger yield that would result from many more transactions under a lower rate, you have a chance of hitting.

Whether the administration takes that advice has far-reaching political and economic consequences. As chairman of the Senate Finance Committee, Long is offering the president a way out of the tax corner he has painted for himself. Oddly, the key to that escape is accepting Long's conviction that reduction of capital-gains-tax rates could increase, not decrease, total revenue.

The reason: Long approaches Carter's complaints that the House-passed tax bill provides excessive tax relief to upper brackets by proposing more to the poor, not less to the rich. But to keep this largesse within budgetary strictures, Long must persuade the administration that the bill's capital-gains section will not lose revenue.

Nothing so illuminates the disparate political styles of these two sons of the Deep South. The president has grudgingly but steadily given ground for 18 months on his rigid insistence that tax changes must soak the rich. Having lost control over the House version of the bill, he now faces a choice between a politically disastrous veto or a new set of principles.

The president's would-be rescuer is not bound by doctrinaire adherence to income redistribution. Long likes tax cuts for everybody, from millionaires on down. Whereas the president has cram-course knowledge of the Internal Revenue code gained since January 1977, the senator has spent 25 years exploring its infinite varieties. "He is a genius at innovation," says one administration official.

Genius begins this time by giving the House bill a less plutocratic, more populistic look, not by taking from the rich buy by giving to the poor. Long wants to compensate taxpayers in all brackets for new Social Security tax increases, and then give enough help for taxpayers making less than $25,000 to compensate for inflation.

But the Senate budget resolution limits total tax reductions to $19.4 billion. How can Long cut individual taxes so generously and still have money left over for the deep capital-gains reductions that he wants? Whereas the Treasury estimates a $1.9 billion revenue loss from capital-gains taxes alone in the $16.3-billion House bill, Long wants much more radical cuts. He would, in effect, put the top capital-gains rate at 19.5 percent, compared to the present 49 percent.

He would soften this with populistic flourishes. He would crack down on the two remaining millionaires who escape all tax liability through shelters. He would increase the capital-gains rate to 30 percent for millionaires escaping other taxation. He might even seek part of that old standard of tax reformers: taxing capital gains at death.

But mostly, Long would use a sharp pencil to correct the Treasury's horrendous but artifical revenue loss. The chairman cogently argues that locked-up capital assets - securities or real estate - would be unlocked by his proposed low tax rate. The present 49 percent of nothing equals nothing; his proposed 19.5 percent of something equals a hefty revenue gain, got a loss. In other words, aim ahead of the flying ducks.

The Treasury insists it cannot do that for fear of impeaching its credibility. But Treasury officials admit this: If Long actually got through the perils of the Senate the bill he is mentally concocting, it would be glorious for investors and populistic enough for Jimmy Carter to sign.

The notion of Long rescuing the president is not so bizarre. After he and his wife dined at the White House last month, the senator rose at dawn the next morning the rewrote his speech to the National Press Club., deleting any anti-Carter taint. Nor has he joined House members, either privately or publicty, in attacking the president's stiff-backed attitude. Instead, Russell Long is trying to spring Carter from his homemade tax trap by making plutocrats and paupers both happy - if only the president's tax estimators will let him do it.