In the ordinary pattern of American politics, the president's party usually gets clobbered in the final off-year congressional election of a presidential term, when a lame duck administration has been in office for six years. By then, the voters are restless and receptive to the out-party.

In the 1950 election, late in Truman's administration, the president's party lost five Senate seats and 29 House seats to the opposition Republicans. The twilight of the Eisenhower era, 1958, saw an epic Democratic comeback, with Republicans losing 51 seats in the House and eight in the Senate. And in the Watergate election of 1974, six years into the Nixon-Ford regime, though something of a special case, Democrats ousted 46 Republican congressmen and five GOP senators. In all three elections the off-year congressional gains for the opposition set the stage for recapture of the White House two years later.

This year, even the arithmetic seemingly helps the Democrats. In the Senate, the heavily Republican class of 1980, which swept into office on Reagan's coattails, is up this year. Seven GOP freshmen squeaked in with less than 52 percent of the vote, several in heavily Democratic states. Eighteen Republican incumbents must face the voters, compared with just nine Democrats.

By the logic of history and statistics, therefore, the opposition Democrats ought to have a good shot at winning a net gain of four seats necessary to take control of the Senate. But in state after state, Democrats face the prospect of a potentially close race -- and no horse. In the depressed Midwest, where industrial unemployment is high and farmers are in revolt, several Republican incumbents are vulnerable. But in several states, there is no serious Democratic contender: none in Indiana, none in Iowa, none in North Dakota, none in Kansas. And in several others the strongest challenger chose not to run.

There's more than one factor at work, but the main culprit is campaign finance, where the Republican money advantage continues to widen. The Republican Senate campaign committee grossed $83 million in the 1983-84 election cycle, compared with less than $10 million for the Democrats. The Republican committees regularly spend the legal limit, and then some. In most states the Democrats don't. This year, taking all resources into account, Republican Senate candidates will probably outspend Democrats by about three to one.

This imbalance not only guarantees Republican candidates a louder voice once the campaign begins, but more insidiously, it often discourages the potentially strongest Democratic challenger from making the race.

Dan Glickman, the popular Kansas Democratic congressman who seriously considered challenging Senate Republican leader Bob Dole, says, "Dole could have outspent me three or four to one. I calculated that I needed to raise about $3 million to be taken seriously; I might have done it, but I wouldn't have had time to do anything else." Glickman decided not to make the race. In Oregon Rep. Ron Wyden, probably the state's most attractive Democrat, passed up a Senate race for much the same reason.

In New York Republican freshman Sen. Alfonse D'Amato is a far right conservative on every issue except pork barrel projects for the state. In a normal year half the Democratic politicians in New York would be lined up to challenge him. But this year, one prominent Democrat after another decided to pass up the contest, leaving as front-runner for the Democratic nomination consumer activist and writer Mark Green, who says D'Amato will raise more funds than he by at least four to one. D'Amato, taking advantage of incumbency, has locked up many normally Democratic sources of money. Complicating the Democrats' Senate problem in New York is Democratic Gov. Mario Cuomo, who has vacuumed up every spare liberal nickel.

"We try to fund the close races," says Sen. George Mitchell of Maine, who chairs the Democratic Senate Campaign Committee. "We can't give much money to the sure winners and the probable losers. And that's a serious impediment when you're trying to recruit candidates."

Moreover, as Mitchell acknowledges, that strategy denies Democratic long shots the resources to win upsets. "Who knows which candidate might catch fire?" Mitchell laments. "We are forced to make those decisions before the race ripens."

Armed with almost unlimited funds, Republican dark horses come from behind far more often than their Democratic counterparts. In 1980, 13 Republicans won by less than 4 percent; only two Democrats did, both of them incumbents. In the last three elections most challengers who won close Senate races were Republicans.

Some other factors will help Republicans to beat the off-year odds next November: the economy is unexpectedly sunny, and Providence seems to smile on the presidency of Ronald Reagan. But in politics as in life, the Lord helps those who help themselves. And the Republicans are able to help themselves to far more political money than the Democrats.

As long as that remains the case, our political system will have an alarming tilt, leaving the more liberal of the two parties unable to field its best candidates or to compete fully in the political marketplace of ideas.