IT MAKES YOU think -- even in August -- when you learn, within a short space of time, that Americans have contributed $25 million to the contras of Nicaragua, while the Washington shelter for the homeless, a few blocks from the Capitol, took in a couple of bob, to be exact, $200, from a group of summer interns.

The Republican practice of giving alms for arms is well known. Conservative Republicans, as a group, sympathize much more with weapons failures than human failures. They puddle up whenever they think the MX, for instance, is about to die. Their instinct, when confronted with a plane that won't fly or a gun that won't fire, is to give it another hundred million or so and murmur, "There, there."

The contras appeal to them on two levels of their being: They like weapons and they are fighting off the communist invasion, which lurks as a clear and present danger in the minds of all right-thinking right-wingers.

A Mrs. Ellen Garwood of Dallas, Texas, who recently contributed $65,000 for a helicopter for the contras, spoke for all the militants when she said, "We're about to go under unless we keep those freedom fighters going." The grateful contras have named the chopper after her. It will be known as "Lady Ellen."

What a hint to the homeless, if they would only take it. They could be fed from the same hand if they played their cards right. Of course, they never do play their cards right, and that's one of the reasons rich Republicans have no time for them.

Also, let's face it, they smell bad, they drink cheap wine, they have wandering wits and they are losers. Deep in the conservative psyche lurks the thought that the homeless have willfully and deliberately chosen their fate. Had they invested more wisely, diversified their portfolios, they would not be stumbling into the shelter at Fourth and D on these nights of blast-furnace heat.

They are freeloaders; the contras are free-enterprisers. The federal government is going to close the homeless shelter on Aug. 31, giving the homeless a chance to seek alternative solutions -- that is, to pull up their socks.

But the shelter need not be closed. A happy ending is possible, with just a little image adjustment. All the homeless need to do is give their nameless shelter a name, one that would describe its new role as a contra training camp. You can see the Mrs. Garwoods of the country reaching for their checkbooks. For that matter, why not call the place "The Ellen Arms."

The homeless would train for battle in the shelter basement. I suggest that when they are hardened and ready, they call themselves the "Oliver North Brigade," after the commandante in the White House, the Marine colonel on the National Security Council staff who runs the secret war secretly and deserves a little recognition.

Of course, all of this would be announced at one of the most sacred of Washington insitutions, the fund raiser, which would be held at the shelter. That would end the quarrel between Mitch Snyder, who runs the shelter and says it needs $10 million in repairs, and the federal government, which thinks $2.7 million is enough. The committee for the Freedom Fighters Ball (dress, combat fatigues) could settle that in seconds. Republicans are extremely sentimental about refurbishing interiors. They could repair the roof, put in air conditioning, paint the walls with one appeal from William Simon, Jeane Kirkpatrick or some other spiritual leader of the contra cause.

I think the decor should be martial: jungle-chic, say, masses of potted palms with grenade launchers cunningly concealed in their midst. The door prize? Easy. Two weeks at the front as the guest of Col. Enrique Bermudez, the contra commander and commie-killer, with one health- clinic leveling and one raid on a coffee cooperative guaranteed.

I'm not so sure about the evening's entertainment, which should be a demonstration of the military skills learned by the shelter clients. Many of them fight unseen demons a good deal of the time, and might not look too spiffy. Perhaps the CIA, which is excluded by law from administering the public charity recently voted by Congress, might be able to take a hand in motivation and mind altering.

The homeless shelter would become a model which Ronald Reagan last November promised it to be. Those left behind could engage in the voluntarism that is so advocated by the Reagan administration. They could help with the mailings of the contra committee, appeals for more funds to ensure that the American way of life will be preserved in this hemisphere.