Why should you threaten to campaign against your political opponents on an issue on which your stand is unpopular? The question was raised by the move by the American Conservative Union and some 30 other conservative groups to target seven congressmen who oppose President Reagan's proposal for aid to the Nicaraguan contras. From now until the April 15 vote on the issue, the conservatives will concentrate on persuading these seven and several dozen others to change their minds. If they don't, then the conservatives threaten to go into their districts and campaign against them on the issue.

Yet every survey on the subject shows that most Americans today oppose contra aid. Their stand has been affected little if at all by the president's initiative; in fact, there is little sign of change in opinion on American policy in Central America since before Mr. Reagan took office. Americans don't like communists, and they believe that what goes on in the Western Hemisphere is of special importance. But they seem exceedingly wary of any increased American involvement in the region. Given all that, you might expect the opponenets of contra aid would invite the conservatives into their district, and that the conservatives themselves would keep mum and hope lobbyists could quietly eke out a congressional majority April 15.

But the conservatives, like the president, plunge ahead. One reason is that they believe in the cause. Another is that they think they can change voters' minds. They claim a survey by pollster Arthur Finkelstein shows that, given enough information, most voters will support contra aid. They think that is especially likely in the districts of their seven targets, most of which are either Republican or southern.

Before you scoff at all this, remember tat similar tactics were used, with great success, a dozen or more years ago by activists on the other side. In 1972 environmentalists targeted House Interior Committee chairman Wayne Aspinall and beat him; Equal Rights Amendment advocates backed a successful primary opponent to anti-ERA Judiciary Chairman Emanuel Celler. The message got through. Before, politicians might have thought it was politically risky to be identified with environmentalism or ERA; for some years afterwards they thought it was risky not to. Now it is conservatives who are taking an issue on which their stand seems unpopular and using it against candidates who seem strong. It is a gamble, but like all gambles it could pay off. Nothing so concentrates the mind of a politician as the defeat of a colleague he thought was safe.