THE BEST THAT you can say for the Democrats' trade bill is that it's not meant entirely seriously. The authors intend it primarily to be a warning to the administration that it is going to have to give the trade deficit, and the tidal wave of imports, more than the cool and casual attention they are now getting. Otherwise Congress may do something really atrocious, suggest the three principal sponsors -- Chairman Dan Rostenkowski of the House Ways and Means Committee, Rep. Richard A. Gephardt, and Sen. Lloyd Bentsen, the ranking Democrat on the trade subcommittee.

Something really atrocious, like what? Like passing the Rostenkowski-Gephardt-Bentsen bill.

It's true that American trade is grossly out of whack, and the imbalance is causing a spreading loss of jobs in American industry. The principal reason for it is the dollar's exchange rate, now about 35 percent higher than its value in traded goods. Doing something serious about the overvalued dollar means raising taxes, reducing the federal budget deficit and cutting the country's excessive dependence on foreign borrowing. That's hard to do. It's more satisfying to flail around wildly and threaten severe punishment for people who live far from here and do not vote in American elections. That's the spirit of the Rostenkowski-Gephardt- Bentsen bill.

The bill would put a 25 percent tax on all imports from countries that do heavy trade with the United States and that sell 65 percent more here than they buy. What about Saudi Arabia? You will be enchanted to know that only "non-petroleum" imports count. There's a total exemption for OPEC, which has done so much to recommend itself to American affections over the years. It's okay to tax the foreign cars that Americans buy, or clothes, or foodstuffs, according to R-G-B. But a tax on gasoline? Don't be absurd.

The bill would immediately hit imports from four countries: Japan, Taiwan, Brazil and South Korea. Two of those countries, Brazil and South Korea, are carrying tremendous debts to American lenders. There's only one way that they can earn the dollars to pay those debts, and that is to sell to the United States. The real target of this bill is of course Japan. But even if Japan were dropped totally out of the trade accounts, the United States would still be running a trade deficit with every major region of the world, and the total would still be unacceptably large.

It's a disastrous bill, but R-G-B are quite right about one thing. The Reagan administration cannot afford to keep shrugging off this rising trade deficit with vague suggestions that things will work out. Left to themselves, things will work out unpleasantly. But responding with a tax on imports is like responding to a dam burst with a tax on water consumption in the valley. The trouble lies a little farther upstream.