Fed up with the lame-duck session? Blame George III

You don't like the lame-duck session of Congress? Blame it on the Founding Fathers.

So fearful were they of royal usurpation two centuries ago that they formulated the ''separation of powers.'' Nearly 200 years later, this has often meant that President Reagan pulled one way while Congress pulled another. Things got so confused in the final hours of the departing 97th Congress that government workers were directed to report to their jobs whether the Treasury technically went broke or not.

Some observers think no other country could operate under the unique American system. Most democracies have parliaments where the legislative and executive branches are joined at the top in a cabinet that dominates the former and directs the latter. The American system works suprisingly well, many historians conclude, though it's not always possible to tell why. The requirement for two-thirds Senate treaty approval kept the US out of the League of Nations in 1920 and possibly led to World War II. On the whole, however, America has made its odd system work and has the highest standard of living in the world.

Mr. Reagan called the lame-duck session contrary to the advice of some senators. He wanted the MX missile among other things, but the House rejected it 245 to 76. That would almost automatically bring a national election in England, Canada, or other parliamentary countries. Here, the President continues his fixed term.

It is believed Reagan's prestige is damaged somewhat, but a midterm sag is notable for most recent presidents. Some observers say they think they discern a trend: No president since Dwight Eisenhower has served two terms, discipline of political parties has shrunk, the percentage of voters at the last five presidential elections has declined, and the clout of political action committees has grown.

To change the US Constitution would require a national convention, which nobody wants, or laborious constitutional amendments. The American Political Science Association in 1950 urged stronger parties and proposed a party council of 50 members. Nothing came of it. President Reagan is learning again what separation of powers means.