THE POLITICS of the new thrift has a firm grip on Montgomery County, and you can see the effect of it in the primary election campaign. There are six candidates for county executive - three Democrats and three Republicans - and all six have crowded into precisely the same point on the spectrum.All six favor rigorous fiscal control, tough (but responsible) cuts in budgets, and a most sincere regard for the taxpayer's dollar. Prosperous and sophisticated, Montgomery County's electorate is once again an accurate indicator of upper-middle-class attitudes that are disproportionately influential in this country.

Montgomery's voters have always been demanding in their standards for public services, and recent budgets have been rising much faster than the inflation rate - while school enrollments have been declining. This disparity is now generating a revolt, and there is no doubt that the six candidates have read the signals accurately. An organization called TRIM needed 10,000 signatures to force 2 referendum on a mandatory reduction in property taxes, in the manner of California's Proposition 13. As it turned out, TRIM rapidly got more than 15,000 signatures, and that question will be on the November ballot.

Politics in Montgomery County is like a ride on a boat in which the passengers rush simultaneoustly from one side to the other, all seized by the same sudden impulse. The present campaign is absolutely different from the last one when the single dominating issue was growth. That meant growth of population, growth of commerce, real-estate development and zoning. Four years later, especially among Democrats, the thought has occurred that a bit of growth, carefully regulated, might not be a bad thing. It might, in fact, help pay the county's bill.

The bland style of the present campaign is also a calculated reaction to the vehemence of 1974. The Democratic Party that year made the mistake of endorsing a candidate who was overwhelmingly beaten in the primary by a crusader for sharp restrictions on growth; the crusader was then beaten, in turn, by the Republican, James Gleason. Mr. Gleason carried over the abrasive temper of the campaign into the conduct of the office. Now he is retiring, and most of the six candidates to succeed him have made much of their intentions to end the constant biting and scratching among the executive's office and all the other local and regional agencies - the county school board, in particular.

Since there is no substantial difference of policy among the candidates, how are voters to make up their minds when they vote in the primaries on Tuesday? Experience in public office is one qualification. Among the Republicans, Richmond Keeney is a former county councilman who now serves on the county planning board. Gerald Warren was an assistant county attorney for three years, and Albert Ceccone has held no previous office. All three Democrats have had substantial experience. Royce Hanson, chairman of the county planning board, has been in public life longest - although, in the present climate, it's not clear whether that will help or hinder him. Charles Gilchrist is a state senator. John Menke is currently on the county council.

The election for the next Montgomery County executive has, in a sense, already taken place. The direction that the office will take is already fairly clear, and the break with the Gleason administration is already ensured. In short, Montgomery County has changed its mind, and it is evidence of how strong the new attitude is that, among six candidates, there is only one platform.