COUNTING the people of the United States has gotten more difficult each decade since the Founding Fathers made the task a constitutional duty. The millennial census, in the year 2000, will be a kind of Olympics of head-counting. To gear up, the Census Bureau is poised, five years early, to dive into new technology and statistical methods. It would dearly like to avoid the controversy that flared after the 1990 count, when charges of significant undercounting ignited political skirmishing and legal action. The Bush administration was suspected of political motives when it chose to go with a tally that excluded later statistical projections intended to adjust for an undercount of minorities. Those minorities tend to be concentrated in urban areas and historically are more likely to support the Democratic Party. A lawsuit against the federal government for using the unadjusted count, brought by New York and other populous states, is still working its way through the courts. But the Census Bureau's desire to leave such battles behind has run into another partisan thicket: congressional Republicans' determination to cut or slow most federal budgets (including that of the Census Bureau) in order to meet their 2002 target for eliminating federal red ink. As budget items go, however, the census is small pickings - though the 1990 count was the most expensive ever, at $2.6 billion. The Republican budget cutters make a fair point when they say the Census Bureau should take its share of trimming along with every other agency. But Republicans lose that logical thread when they continue to balk at the bureau's use of statistical sampling to estimate populations, often minority, that are hard to get at by mail. This is a practical necessity, especially at a time when growing numbers of people fail to mail back their census forms. It may also be a fiscal necessity, since one of the bureau's largest 1990 outlays - 20 percent of the total - went to hiring troops of enumerators to visit households that didn't respond to mailed forms. If the cuts go too deep now, the Census Bureau will be kept from testing new methods that will save money - and trouble - ahead. Budget balancers should keep that in mind. And the census's host of beneficiaries, from scholars to marketers to, yes, politicians, should themselves gear up to safeguard its thoroughness and integrity.