Washington — The two-year process by which America picks its president -potentially the most powerful man in the world - has begun. In Washington President Reagan is deciding whether to run again. In Sacramento seven potential Democratic candidates exhort the party faithful at a state political convention.
Other democracies pick political chiefs from the party hierarchy in parliament. The United States winnows down the nomination in a unique process - a kind of media free-for-all.
The presidential selection process will now go into every state with a rough scoreboard set up by the press and television media, supported by frequent public opinion polls. The system is such that obscure candidates can gather strength during 1983 and thunder down to the rival party conventions next year, at which the official nominees are actually selected.
Few Americans, let alone foreigners, understand the process. It is complicated because the president is part prime minister and part monarch, and shares responsibility with the two-part legislature, which itself may also be politically divided, as it is now.
''We have created a nearly impossible presidential job description,'' says political scientist Thomas E. Cronin. ''We give our presidents too much to do and too little time in which to do it.'' This, he thinks, creates ''unrealistic expectations - the presidency, an already impossible job, has become sometimes the sole focus of our hopes and aspirations as a nation.''
The press and news media have already picked a ''front-runner'' on the Democratic side. With the withdrawal of Sen. Edward M. Kennedy (D) of Massachusetts, it is former Vice-President Walter Mondale, selected at the New York convention in 1976 as Jimmy Carter's running mate.
Mr. Mondale's rather subdued speaking style has handicapped him until now, but at the Sacramento gathering last weekend he let himself go and got an ovation.
He declared that if he were president he would immediately sit down with Soviet leader Yuri Andropov and demand ''a verifiable mutual freeze on nuclear weapons.''
Sen. John Glenn (D) of Ohio comes second on several preliminary opinion polls. As the race starts, half a dozen other Democratic hopefuls are making their personalities and opinions known.
Mr. Reagan, in spite of a recent midterm slump that normally comes at this time in an incumbency, can almost certainly have a presidential renomination if he wants it. He is under pressure from other Republicans to make his decision known before the contest goes much further.
Democratic Party leaders have made new rules consciously designed to shorten the season in which states hold primaries and pledge delegates to certain candidates. This is the biggest change in the 1983-84 contest. In 1976 five weeks intervened between the preliminary Iowa caucuses and the first state primary in New Hampshire. Next year, however, only eight days intervene between Iowa and New Hampshire. And a week after New Hampshire, under tentative schedules, there will be primaries in 15 other states.
This formidable concentration will make coverage difficult and give advantages to front-runners - those candidates who have already established themselves to some degree. This is is said to be the purpose of the new rules drawn by those who believed that too many candidates were presenting themselves and that the system had gotten out of hand.
Another big prospective change is the AFL-CIO's plan to give an early endorsement to a candidate, presumably a Democrat, before the so-called beauty contests are complete. The labor convention meets next December. Formal endorsement of a candidate and party as early as that could put a new face on politics, perhaps aligning the US trade-union movement more specifically with a particular political party.
Candidates are closely watched in the winnowing process and may be destroyed by a gaffe or an unpopular position. Preliminary poll ratings give tenuous leads on the Democratic side to Messrs. Mondale and Glenn, and a group of hopefuls follow: Sen. Gary Hart of Colorado; Sen. Alan Cranston of California; former Gov. Reubin Askew of Florida; Sen. Dale Bumpers of Arkansas; George McGovern, the 1972 candidate; and Rep. Morris K. Udall of Arizona.