The Teflon is still in place, judging from the polls, but there are increasing doubts among his supporters that President Reagan has the toughness they supposed -- or that effective leadership required in these times.

As the latest terrorist incidents in Lebanon and El Salvador slip into the history books, conservatives are questioning what ever happened to Reagan's 1981 promise of "swift and effective retribution" for attacks on American citizens.

The sad truth is that hundreds of American lives have been taken by terrorists in the past 41/2 years, and no one has been punished. After picking up a cheap win on Grenada in 1983 and lobbing a few naval shells into Lebanon in 1984 to cover the withdrawal of the Marines from that misguided deployment, Reagan apparently has forsaken the threat of force in international relations.

Now the conservative Heritage Foundation, a source of people and ideas for the Reagan administration, has raised the embarrassing question of Reagan's unilateral disarmament in domestic politics. It asks how deficits have reached record levels under Reagan without his systematic use of one of the great constitutional powers any president enjoys: the veto.

It's a pertinent question as Congress returns and Reagan begins to involve himself again in the unsettled issue of the fiscal 1986 budget.

In a policy paper issued last week, James Gattuso and Stephen Moore of the Heritage Foundation staff put the issue in two succinct paragraphs:

"Seldom has a president adopted a tougher stance with a Congress than has Ronald Reagan in the first months of his second term. He has vowed to veto congressional efforts to raise taxes or pass budget-busting spending bills. He even has taunted Congress to 'make my day' by passing a tax increase. The message is clear and welcome: Ronald Reagan would appear to relish vetoing the actions of an irresponsible Congress. He seems to recognize that the veto is a president's trump card.

"The problem is that, despite the tough talk, Reagan actually has been very timid in playing this trump thus far in his presidency. This apparent aversion to vetoing may seriously impair Reagan's ability to prod Congress to act responsibly, particularly in slashing federal spending -- where few major victories have been won since 1981."

Reagan has not shunned the veto. Early this year, he vetoed an emergency farm-credit measure. His support was so evident that the House Democratic leadership did not even attempt an override.

But in his first term, Reagan used the veto only 39 times -- barely half the annual average of all presidents during this century and even further below the average for presidents who have faced a Congress partly or entirely controlled by the opposition party.

White House chief of staff Donald T. Regan has argued that "there has been nothing mammoth to veto." But that statement is hard to reconcile with the administration's repeated assertions that Congress is to blame for the massive deficits facing the country. The Heritage study asserts that "Congress has presented Reagan plenty of bills of dubious merit which he chose to sign rather than veto." The examples it cites range from farm subsidies to education and grant-in-aid programs. Cumulatively, they explain part of the climb in federal spending above the levels Reagan had requested.

But the Heritage authors do not acknowledge that much of the deficit they deplore stems directly from the tax reductions Reagan pushed through Congress at the outset of his presidency. Nor do they examine the point made by congressional Democrats: that Congress has rearranged spending priorities but not increased overall appropriations beyond budgeted levels.

Still, the thrust of their criticism is valid. In domestic policy as much as in foreign affairs, Reagan has tended to huff and puff -- but rarely to blow the house down. He is observing the limits of the arms-control treaties he once denounced and moving toward a summit conference with the leader of the Soviet Union, a symbol that detente is once again back in fashion.

He would rather threaten retaliation against terrorists than take concrete actions to punish those who kill Americans. And he would rather complain that Congress won't give him new item-veto power than use his existing veto power as predecessors from Franklin D. Roosevelt to Dwight D. Eisenhower and Gerald R. Ford did.

So far, Reagan's rhetoric has convinced people that he is a man with the strength of his own convictions. But as time goes on, more than the conservative ideologues will begin to suggest that this man is something of a paper tiger.