In politics, you go hunting where the ducks are, and this year the Republicans and Democrats are trying to poach on each other's preserves. The GOP is going after Hispanic voters who overwhelmingly register Democratic, and Democrats are aiming at the economically troubled Farm Belt.

The Democrats began a three-week radio advertising campaign last week on 180 stations in nine farm states -- Iowa, Kansas, Minnesota, Missouri, Nebraska, Oklahoma, Wisconsin and the Dakotas. The theme of the ads, narrated by actor Martin Sheen, is that farmers have been betrayed and abandoned by the Reagan administration and that voters should "send the administration a message" by electing Democrats.

Iowa Democrats also prepared an ad to greet Vice President Bush's scheduled visit to Waterloo: "We hope the news you are bringing here is as good for us as the news you took to Saudi Arabia was for the oil and gas interests in Texas."

The Republicans are testing print and radio ads in California, Texas, New Mexico and Arizona prior to Mexico's Cinco de Mayo (May 5) celebration, on the assumption that Hispanic support for President Reagan in 1984 gives them an opening. The Hispanic vote in those states could also be crucial in Senate and gubernatorial races this year, and the GOP plans to spend about $300,000 on its Hispanic project by Election Day.