The most powerful force in this November's congressional elections appears likely to be the inertia of incumbency.
A race-by-race review of all 435 congressional district contests, at the Labor Day start of the fall campaign, shows relatively little margin for change in the Capitol Hill power balance.
Professionals in both parties, in a Monitor tally, rated 72 races as very close or likely to change hands. Of these, 33 are for seats now held by Republicans, 29 for seats held by Democrats, and 10 for newly created districts. In at least four of the close races, incumbent Democrats will face off against incumbent Republicans in states losing seats by redistricting.
Thus on a strictly numerical basis, there is a slight Democratic advantage for this fall, with two months of campaigning to go.
This finding appears to confirm estimates of leaders in both parties that, despite deep unemployment and a long delay in economic recovery, the House races this fall could show less than the average losses for an incumbent president's first midterm election. A president's party has averaged about a 15-seat loss in recent years, ever since Dwight Eisenhower's Republicans lost only two seats in 1956.
The reverse side of the small number of races rated close (72 of 435, or 16 percent) is that in over four-fifths of the races, one candidate - usually the incumbent - is given a clear edge.
This puts 1982's elections in the same category as most other midterm races since World War II, as basically status-quo elections. Even in 1980, 83 percent of all incumbents in House races were returned to office. Despite commentary calling 1980's results exceptional, this was exactly the average of all House races since 1946. Ninety-one percent of all House members in 1980 who chose to run again were reelected - again exactly the average since 1946.
The ''micro,'' or race-by-race look at the congressional campaign shows little evidence of a Republican bonus this year from redistricting, from any conservative trend, or from the Sunbelt's gain of congressional seats.
Of the 21 newly created districts, the GOP rates itself ahead in 10 races, the Democrats ahead in six, and five even. The Democrats rate themselves ahead in 11, the Republicans ahead in six, with four even.
By their voting records, the incumbents in close 1982 races are somewhat more conservative than Congress as a whole. The 60 incumbents fighting for survival averaged 35.3 percent in the Americans for Democratic Action (ADA) liberal/conservative scale in 1981, compared with the House ADA average of 40 percent.
Overall, President Reagan's basic appeal in the close-race districts does not appear to be a factor. Mr. Reagan averaged 51.9 percent in those districts in 1980, according to the Monitor analysis, about his national average.
The most notable feature of the close-race districts in 1982 is that the races were close in those districts last time. Of the 58 districts with essentially the same political turf as in 1980, the winners two years ago averaged 59 percent - or less than the 60 percent cutoff line for ''marginal'' districts, and well below the 67 percent average for all House election victors.
By region, three more Democratic incumbent seats than Republican seats are rated at risk in the combined South and West; seven more Republican seats than Democratic are at risk in the Midwest and East. This hardly reflects a 1982 Sunbelt advantage for the GOP.
In the South, the split is even - 10 Democratic and 10 Republican seats rated close, with three close new-seat races. In the West, five Republican incumbent seats, eight Democratic, and six new seats are rated close. In the Midwest, eight Republican seats, four Democratic, and one new seat are rated vulnerable. And in the East, 10 races for GOP incumbent seats and seven for Democratic seats now look close enough to go either way.
In 1980, the Republicans were able to pick up seats by performing remarkably well in so-called ''open seats'' races, for seats vacated by retirement or an incumbent's seeking other office.
In 1982, open seats do not appear particularly promising to either party. Of the races for 16 open seats being vacated by Republicans, about 5 are rated close at this stage of the election, both sides agree. Of 19 Democratic open seats, again about 5 are rated close contests.
Some Democratic professionals have been saying the GOP's 1980 freshman class may be vulnerable in 1982. The group does appear, in both parties' race-by-race ratings, as somewhat more endangered, if only because there are more such seats. About a third of the 51 races for seats won by GOP freshmen in 1980 are rated close this fall. The Republicans rate 15 of their 1980 freshman races as very close, the Democrats rate 18 of the same races close.
Democrats won far fewer GOP seats in 1980 - just four, three by defeating GOP incumbents or one in races for Republican open seats. They held 18 Democratic open seats with Democratic freshmen victories. This time, also, about a third of the 22 Democratic freshmen seats of the class of 1980 appears at risk. The Republicans call seven of the races close, the Democrats four.
Overall, the ratings of the fall races by Democratic and Republican professionals are remarkably similar. Democrats say they will likely gain only about 10 seats. Republicans still claim an outside chance to pick up a seat or two, or lose possibly 15 to 20 seats.
The similarity of such ratings by the two parties may come as a surprise to the general public. But the professionals look basically at the same data. They know the candidates and the districts well. They tend to discount somewhat the ''macro'' or national factors such as presidential popularity and unemployment, except as bearing on particular districts.
Some political analysts, looking primarily at national factors such as party identification by voters, anticipate swings of as many as 40 seats to Democrats after Nov. 2.
Party professionals concede that broad economic trends could help many of the close races cut more to one party's advantage. But they also agree that the Republicans' edge in finances and firepower - including potential presidential visits - tends to offset any potential Democratic edge in national factors.