The 10 biggest-spending political action committees gave $5.8 million to federal candidates in the 20-month period that ended Aug. 31, and had enough left to contribute more than $5 million more by Election Day. That would work out to $84,000 a day, Saturdays and Sundays included, until Nov. 4.

These figures come from a Washington Post tabulation of Federal Election Commission data on the top 10 PACs among the 1,650 which together contributed roughly $30 million between Jan. 1, 1979, and Sept. 1, 1980.

Most of the money went to House and Senate candidates for primary, runoff, and general elections and to party committees for distribution to those same candidates. A relatively small share went to candidates for presidential nominations.

The Sept. 1 figures are anything but final. Large sums continue to flow into PAC treasuries, even though, if history is a guide, a significant amount will remain in the tills on Nov. 4.

The combined cash on hand reported by the top 10 PACs as of Aug. 31, $5.3 million, was 17 percent less than at the same point in the 1977-1978 election campaigns. But the $5.8 million spent by the same units -- arms of various business, labor, professional and agricultural organizations -- was 25 percent more than the 20-month figure for 1977-1978.

Pacing the current spenders, all of which have large stakes in various pieces of legislation and pending administrative actions, is the PAC of the National Association of Realtors. It listed contributions of $991,265, up 51 percent from two years ago. The NAR has been battling a bill intended to put enforcement teeth into the fair-housing law.

Ranking second, with $833,835, was the political arm of the American Medical Association, down 17 percent. In third place, with $712,041, up 46 percent, was the United Automobile Workers unit, one of five union PACs on the list.

The other groups whose PACs made the Big 10 list and the percentage change from two years ago: National Automobile Dealers Association, $663,008, up 65 percent; AFL-CIO, $556,727, down 9 percent; the Carpenters and Joiners of America, $452,058, up 119 percent; the United Transportation Union, $423,819, up 3 percent; Associated Milk Producers, Inc. (AMPI), $406,389, up 19 percent; the International Association of Machinists, $396,430, up 32 percent, and the National Association of Life Underwriters, $377,642, up 68 percent.

Many PACs hold back their contributions until the crucial last few days before an election to get a surer sense of who the winner may be and contribute to him, or, if a race is close, hedge their bets by giving to opposing candidates. In another form of political switch-hitting, some PACs that have bet on the wrong horse show generosity to the surprise winner by contributing to him after the election.

As in many past years, the largest single treasury, $1,388,801, belongs to AMPI's political unit, which goes under the acronym C-TAPE. With 30,000 members in 20 states, AMPI, of San Antonio, Tex., is the biggest of the three major diary cooperatives. About 12,000 AMPI members contribute to C-TAPE, authorizing average deductions of $45 from their monthly checks for milk, according to C-TAPE administrator J.S. (Sam) Stone.

Running a close second to C-TAPE, with $1,191,496, is the auto workers' PAC. Next are the realtors, $683,291; the physicians, $682,566; the machinists, $421,411; the auto and truck dealers, $368,945; the railroad workers' transportation union, $293,935; the life insurers, $159,165; the carpenters, $58,354, and the AFL-CIO, $35,963.

The labor PACs accounted for most of the decrease in cash on hand in 1980 as compared with 1978. The AFL-CIO had 92 percent less, the machinists 64 percent, the carpenters 56 percent. By contrast, the auto and truck dealers had 22 percent more money on Aug. 31, while C-TAPE had 17 percent more.

While both gathering and spending large sums, the dairy co-ops generally also have kept their treasuries brimming with cash. C-TAPE's Stone, in a telephone interview, said that the PAC has $1.1 million to $1.2 million in certificates of deposit that have paid interest in 1980 of 11 to 17 percent. i

He shied away from spelling out why C-TAPE has ended up one election after another with huge sums unspent. But between now and Nov. 4, he said, C-TAPE will spend "a substantial amount" on activities.