"DIVISIVE, NON-USEFUL, expensive and extraordinarily irritating." That's what Democratic National Chairman Charles Manatt has to say about the straw polls that Democrats all over the country have been taking. He could just as well be talking about the whole presidential selection process, which is heating up 12 months before the Democrats meet in San Francisco and 15 months before November 1984. There's something ludicrous about a contest that takes almost as many years to conduct as the ultimate winner gets to serve.

Still, the straw polls do tell us something. Last month's victory for Sen. Alan Cranston in the Wisconsin straw poll showed there's still plenty of support among Democratic activists for the candidate who seems the most dedicated to disarmament and the nuclear freeze. The more recent victory of former Florida governor Reubin Askew in a straw poll at a Manchester, N.H., city Democratic picnic shows that there's support for a candidate who campaigns person-to-person despite being unknown nationally and little known here in Washington. Mr. Askew won 1,066 votes at the picnic compared with 34, 23 and 14 for other candidates.

Don't jump to the conclusion that the other candidates have no support in New Hampshire; some claim, plausibly, that they didn't know about the picnic at all. What's significant is that Mr. Askew was able to motivate some 800 New Hampshire voters (some 200 Askew voters were from other states) to make a special trip on a muggy summer day. It couldn't all have been the free tickets he handed out. It's a significant showing in a state where only 111,000 people voted in the last Democratic presidential primary.

Mr. Askew has an attractive theme: he is trying, he says, to win the nomination and election without making promises that will make it impossible for him to govern well. His scenario for victory is good showings in the Iowa caucuses and in New Hampshire, followed by a victory in Florida. His problem, and that of other candidates who begin the race unknown nationally, is that Democrats may be wary in 1984 of nominating a candidate of whom they know no more than what they knew about Jimmy Carter in the spring of 1976. Such candidates are right when they say that the press has an obligation to tell voters enough about them to make an informed decision. But will voters get to know an unknown well enough in a few weeks, this time, to be comfortable in entrusting him with a presidential nomination?