The chances are that House Democrats will win their initial battle over contra aid. But the chances are that further along, they could lose the war. Presidents don't quit easily, and Democrats eager to collaborate with a popular president wait in the wings.

On March 19, the House will vote up or down -- no amendments -- on the administration proposal to send $100 million to the counterrevolutionaries, the bulk of it for military aid.

Rep. David E. Bonior (Mich.), leader of the Democratic Task force on Nicaragua, says he is confident of the outcome on the 19th. He says, "If we can't do it on this issue, I don't know where we could do it."

President Reagan and his henchman Patrick Buchanan have framed the issue in an irresistibly ugly way for the Democrats. Buchanan, with his customary gentleness, told them they could vote either with Reagan and the resistance or with Daniel Ortega and the communists.

A Reagan victory, says Rep. Tony Coelho (Calif.), chairman of the Democratic Congressional Campaign Committee, would be "a ratification of Buchanan's red-baiting tactics." Actually, the blast steadied and unified the Democrats. Two of three House committees have said no to Reagan.

Buchanan, the White House communications director, did something even more destructive than trashing the patriotism of the Democrats. He compared Nicaragua to Vietnam.

If there is one thing the dimmest Democrat understands, it is that venturing onto the path that could lead to a replay of the most unpopular war in history is not a good idea. The president keeps saying that he has no plans to send in U.S. troops, but hints he may have to if Congress won't fund the contras to stem the "sea of red" that could be lapping at U.S. shores.

Vietnam may be to the president "a noble cause"; to the Democrats, it is sheer nightmare: 525,000 Americans under arms, America's streets and campuses boiling with protest, a party split, a presidency lost.

Victory on the field in Nicaragua is out of the question, says Bonior, even if we send $100 trillion. The people have not risen up. The most optimistic guess of contra strength is 25,000, which is no match for the Sandinistas' 60,000 seasoned troops.

The Democrats have not only logic on their side, but also the public, as Reagan's pollsters vainly tell him. The country does not want to overthrow Daniel Ortega; it isn't particularly interested in having him say "uncle."

So the prospects are that they will gather their courage and stand up to Reagan on the House floor, no matter what he says next Sunday in a nationally televised address.

The expectation is that he will set aside the sandpaper for the silk. He will appeal for bipartisanship and sing the siren song of compromise as theme music for an eventual victory.

The Democratic collaborators are already lining up, ready to lend a hand and receive the celebrity that comes to those who break with the party on a major issue, and enter into heady parleys at the highest levels of government.

The precedent in this regard was set by Rep. Les Aspin (D-Wis.), who set himself up as a major player by throwing his support to the MX missile. He bragged on the floor about extracting concessions -- yet to be seen -- from the president and got himself no end of attention.

Being a Democratic negotiator with the White House on something that Reagan cares about is to real negotiation what building with Lincoln Logs is to the construction of a Trump Tower. But House members, lost in the pack, have to distinguish themselves the best way they can.

Last June, Rep. Dave McCurdy (D-Okla.) won the limelight by defecting to the president on the matter of $27 million in "humanitarian" aid to the contras. He made his defection contingent on White House negotiations with the Sandinistas. None have occurred, but McCurdy is waiting to take center stage with a new compromise.

McCurdy is working with Elliott Abrams, the angry assistant secretary of state for Central America, who has been almost as vehement as Buchanan on the question.

For Reagan the action on March 19 is major because he is obsessed with not having a communist revolution succeed on his watch. Besides, it is the first big vote of the season.

For the Democrats, it is much more. If they vote for the contras again, they will be permanently branded as the wimp party. And they have to face voting against contra aid again and again and again. March 19 is just the beginning.