To sort out Soviet politics Americans need a seasonally adjusted method for interpreting the zigzag of Mikhail Gorbachev.

In fall and winter he typically aligns himself with reactionary forces.But in spring and summer he turns toward reform and compromise with progressive forces.

He has done it again. After five months of alliance with Communist and military reactionaries, Gorbachev rebuffed the hard-liners last month and is now operating in a partnership with his rival, Boris Yeltsin, the Russian Republic leader and champion of the reformers.

Gorbachev's shift reflects a pragmatic concession to the power of republican governments, one that opens an opportunity for American policy.

The West can widen its official dealings with republican and local governments to promote long-term democratic and market reforms.

Gorbachev himself offered the means for doing so, asking Washington for a new agricultural credit of $1.5 billion.

Clearly President Bush wants to say yes, but Washington should also spread its bets among the contending forces - not abandoning Gorbachev but moving to expand parallel connections at all levels of the fragmented Soviet power structure.

Here are some steps that should be taken:

- Amend the 1990 legislation establishing the Support East European Democracy program to include the Soviet Union, so that the United States can help train mayors and city-council members in the art of democracy. Those reformers confront the hard-line party apparatus and lack experience in governing.

- Invest in building democratic institutions in the Russian Republic and other republics.

With only $300,000 the National Endowment for Democracy through the National Democratic Institute has run seminars for Soviet officials on budgets, taxes, services and coalition-building. Some 200 city officials have begun this training, but 20 times that number need help.

- Take advantage of glasnost's daring spirits such as the TV journalists of the popular program "Vzglyad" who were thrown off the air last December. They are trying to organize alternative TV and radio outlets to compete with the censored state media.

Seed money and secondhand equipment are desperately needed.

- Extend economic advice to republics, regions and city governments that want market reforms.

Until now the Bush administration has been so worried about offending Gorbachev that it has only advised the central government, where the will to create a market is weak.

- Channel U.S. aid, especially food and other humanitarian aid, to republican governments rather than to the Kremlin.

- Gradually expand official relations with Lithuania, Latvia and Estonia, leading to establishment of what Zbigniew Brzezinski calls "institutes," or veiled U.S. embassies, and get the Baltic states accepted as full members at Helsinki II conference.

In sum, the moment has come for bolder American actions.

The object should be to encourage Gorbachev to stay on a progressive track - and to strengthen democratic institutions in case he follows his pattern and swerves back toward a hard line next fall.