"Now what do you suppose he did that for?" Prince Metternich is supposed to have asked on being informed that the Russian ambassador had dropped dead. The Democrats and some Republicans are asking the same question now about Ronald Reagan and his drive for $100 million for the Nicaraguan contras. "They're running a losing game," a leading Democrat says, "and this guy doesn't usually run losing games."

It is a losing game politically -- or at least that is what every Republican political consultant will tell you. Republican National Committee pollster Robert Teeter put something of a pall on a meeting of House Republicans in Baltimore six weeks ago when he told them to avoid contra aid. Reagan pollster Richard Wirthlin, according to The Wall Street Journal's Robert Merry, has been fighting emphasis on the issue and has told political insiders, "The numbers aren't with us, and they aren't changing."

He's right about the numbers. The situation in Central America may have changed in various ways since 1980, but poll results have stayed the same. Americans are wary of any kind of U.S. involvement there. Like so many House Democrats, they fear another Vietnam and want to avoid it at almost any cost.

A standard rule of political consultants is that you emphasize the issues on which the public agrees with you and de-emphasize those on which they disagree. Ronald Reagan, not running for anything but still interested in maintaining political clout, has chosen to violate this rule. The only possible reason is the best possible one: He really believes this is the right policy.

He thinks he has a moral duty to help the contras and get rid of the Sandinistas, even at considerable political risk to himself and his party. He is doing what liberals and conservatives and moderates all ask their presidents to do: spending some of his political capital, risking his popularity, on an important issue on which his stand is politically a loser.

The Democrats, like Prince Metternich, look for less obvious motives. They fasten on Patrick Buchanan's op-ed piece in The Post, which frames the issue as whether "the national Democratic Party has now become, with Moscow, co-guarantor of the Brezhnev doctrine in Central America." The Democrats suspect that Reagan is setting them up to take the blame if the contra aid fails, the contras collapse and the Sandinistas become unshakably entrenched in Nicaragua. But that still leaves them puzzled, because they believe that fear of communism does not resonate with the voters as it did in the 1950s, and that there will not be a Who-Lost- Nicaragua politics in the 1990s to match the Who-Lost-China politics of 1952. In this they are probably right. After all, despite predictions, and despite the grisliness of the flight of the boat people, there has not been a Who-Lost-Vietnam politics in America yet.

This speculation rests on the assumption that the administration will lose the battle for contra aid. Ronald Reagan, sunny optimist, probably hopes to win. He will do so only if he succeeds in changing the national mind on this issue. The House is an exquisitely sensitive barometer of public opinion, as it showed when it switched on this very issue after Daniel Ortega flew to Moscow. That shouldn't have changed the views of sophisticated officials, but it was the kind of dramatic event that changes the views of voters, and House members paid it great heed.

Reagan doesn't have a majority in the House on contra aid going in. But if he succeeds in changing the national mind, he'll get that majority, and the question won't be who gets the blame, but who gets the credit.

If Reagan doesn't succeed in changing the public mind, he will probably lose. He will have used up some of his political capital on what will remain, in the short run anyway, a losing issue. You wouldn't hear much about Nicaragua next November, except maybe from Democrats. This is a serious risk for Reagan. But it may be a risk you'd take if you had a 65 percent job rating and believed that freedom was at stake.