In the most lavishly financed midterm election ever, money often turned out to be one of Tuesday's biggest losers.

The nation's two best-heeled candidates for senator and governor--GOP incumbent Gov. Bill Clements in Texas ($12 million) and Democratic senatorial challenger Mark Dayton in Minnesota ($6 million-plus)--both came up short.

So did Lewis E. Lehrman, a discount drug store magnate who spent more than $11 million making himself a household name, but not quite a governor, in New York.

In the House contests, a surprisingly large crop of 26 GOP incumbents was turned out of office Tuesday despite having outspent challengers by margins approaching, and in some instances exceeding, 2 to 1. And the two richest political action committees, the National Conservative Political Action Committee (NCPAC) and the Congressional Club, also got a low return on their dollar.

This obviously does not prove that money did not matter. Tuesday's winners as a group outspent the losers, as they always do. In the 33 Senate races the margin was 3 to 2.

And Republican Party officials say their losses would have been higher had they not funneled national party money and funds of friendly political action committees (PACs) into key races.

But there were enough winning candidates with more modest campaign coffers to support the argument that what really counts in campaign financing is not having a lot, but having enough.

"There definitely seems to be some kind of threshold point in a campaign, after which you stop getting as much bang for the buck," says Neil S. Newhouse, a political specialist for the Chamber of Commerce, which saw dozens of its well-funded choices go down to defeat.

There was even one race Tuesday where the spending margin was nearly 50 to 1, and guess who won? It was the open seat contest in California's 43rd Congressional District, in rock-ribbed conservative Orange and San Diego counties, where Republican John Crean, a recreational vehicle manfacturer, spent more than $1 million but still finished behind Democrat Roy Archer and Republican Ron Packard who, angered by Crean's primary campaign, ran as a write-in. Archer, a professor of government whose mid-October spending report showed a total of about $20,000.

Big money could not help Cissy Baker, daughter of Senate Majority Leader Howard H. Baker Jr. (R-Tenn.), either. She spent nearly $1 million failing to win an open House seat in Tennessee.

Another handsomely funded candidate with a famous relation, former 1960s radical Thomas Hayden, spent an unprecedented $1.5 million running for a state assembly seat in California. He won, narrowly.

It was a mixed day for millionaires who tried to "buy" seats with their own money. Dayton spent at least $5.5 million of his own in Minnesota, Ray Shamie (R) invested $1.1 million of his business fortune running for the Senate against Edward M. Kennedy in Massachusetts and Rep. Millicent Fenwick (R-N.J.) spent $576,000 in personal funds on her Senate bid. All lost.

Winners who invested heavily in their own causes included Democrat Frank Lautenberg in New Jersey, who gave his campaign $2.7 million, and Democrat Jeff Bingaman, who gave $688,000, in New Mexico.

The most striking development in the House races was the defeat of 26 Republican incumbents, 21 of whom outspent Democratic challengers.

Almost all these races were in districts where the recession has been severe, which suggests that economic factors working against Republicans outweighed their financial advantages. By contrast, only three Democratic incumbents lost, two of whom had been redistricted into fights with incumbent Republicans.

Fourteen of the losing GOP incumbents outspent challengers by $100,000 or more.

These included Rep. Albert Lee Smith (R-Ala.) who put out $336,000 in a futile effort to fend off Ben Erdeich, who spent $135,000; Rep. Thomas B. Evans (R-Del.), who spent $408,000 in losing to Thomas R. Carper, who spent $63,000; Rep. Eugene Johnston (R-N.C.), whose expenditures totaled $316,000, compared with Democratic challenger Charles Robin Britt's $90,000, and Rep. Ed Weber (R-Ohio) who spent $316,000 in a vain effort against Marcy Kaptur, $123,000.

Among the other findings in an examination of the marginal House races:

* Money may well have helped keep Republican incumbent losses down to 26. There were 22 other GOP incumbents in tight contests who held onto their seats and all but one outspent Democratic opponent by amounts ranging from $61,000 to $371,000.

Rep. John Hiler (R-Ind.) outspent Richard C. Bodine $270,000 to $67,000 and won by a slim 51 percent to 49 percent. Similarly, Rep. Denny Smith put $381,000 into his reelection effort in Oregon, compared with $120,000 by his Democratic challenger, J. Ruth McFarland. Smith got 51 percent of the vote and McFarland 49 percent.

* Business political action committees, many of which had helped finance the conservative-Republican victories of 1980, were far less successful this year. In 65 marginal House races where business PACs clearly sided with one of the candidates, the candidate won in only 28, or 43 percent, of the contests.

Organized labor was far more successful. Of the 69 House candidates in tight races receiving $20,000 or more from union PACs, 43, or 62 percent, won. Business PACs, for example, pulled out the stops for Rep. James K. Coyne (R-Pa.), giving him $155,000, but he lost to former representative Peter H. Kostmayer, who got $89,000 from unions. Rep. Phillip Burton (D-Calif.) won with the support of $199,000 from labor, beating Milton Marks, who got $115,000 from business PACs.

* Money may have been a significant factor in the contests for open seats. The Democrats and Republicans split these contests, nine each, with one so close it may be recounted. In 15 of the races, however, the Republican candidates outspent the Democrats and in 12 contests, the Republican edge exceeded $50,000, ranging up to $356,000.

The nation's two richest PACs--NCPAC and Jesse Helms' Congressional Club--were second to none in terms of low return on their investments.

NCPAC spent $2.8 million trying to unseat 14 Democrats; they hit the mark only with Sen. Howard W. Cannon. The Congressional Club fared almost as badly; 15 of its 18 endorsed candidates, including all five in Helms' home state of North Carolina, lost.

Many defeated Republicans this year have either disavowed NCPAC or blamed its negative ads for contributing to their losses. NCPAC Chairman John T. (Terry) Dolan countered that the committee's negative ads kept down its targets' winning margins.

There was plenty of finger-pointing and recrimination by conservatives.

Paul Weyrich, chairman of the Committee for the Survival of a Free Congresss and a leading New Right spokesman, said the national Republican Party, with its massive 7-to-1 fund-raisng edge over the Democrats, should have spent more money on precinct organizing and opposition research, less on "bloated payrolls . . . and defensive media campaigns."