Party political conventions - a uniquely American occasion

The Democratic and Republican political conventions are almost as American as apple pie or the Fourth of July. There is nothing quite like them in the rest of the world. In fact, no other country in the world takes as long as the United States to elect its leader. But no other job in the world is as important or as powerful as its president.

The process of choosing a president takes 10 months. It starts, usually in deep snow, during February in New Hampshire. This is only the first of 29 primary elections across the country, which run through the winter and spring.

In most states, the results of these elections determine which candidates will be supported by the people (called delegates) who attend party conventions. A few states hold what are jokingly called ''beauty contests.'' These are primary elections that simply indicate whom the voters prefer for president and don't tie delegates to any candidate. Still other states hold ''caucuses'' - meetings where Republicans and Democrats decide whom their state's delegates should back at the conventions.

After all these preliminary events come the party political conventions themselves. The Democrats are holding theirs this week in San Francisco. The Republicans will meet Aug. 20-23 in Dallas. At these loud, throbbing gatherings the Republicans and Democrats - amid music, cheers, boos, and much waving of signs and banners - finally cement their selections for president.

Unless the choice of the presidential candidate is absolutely certain before the convention begins, these conventions can be electric with excitement and tension as the nation watches eagerly to see how the votes of the delegates will be cast. A roll call is issued and the dramatic cry goes up: ''Al...a...ba...ma casts its 24 votes for....''

In recent years, however, the decision on who would be the party's nominee for president has more often than not been made far in advance of the convention , when one candidate wins the support of enough delegates in primaries and caucuses to wrap up the nomination. That appears to be so this year with former Vice-President Walter Mondale and the Democrats. President Reagan is sure to be the Republican choice, as parties almost always renominate a president at the end of a first four-year term in office.

At convention time both the Democrats and the Republicans will also make final their choices for vice-president and thrash out what their political platforms will be. The Republicans will choose George Bush, the man now in office. For the Democrats, however, the choice of a vice-presidential nominee could become the high point of the convention. Walter Mondale, the likely Democratic nominee, has already announced that his choice for a vice-presidential running mate is Congresswoman Geraldine Ferraro of New York. She would be the first woman ever nominated for high national office by one of the two major parties.

The platforms are merely lists of issues and stands that the party wishes to take on these issues. It's like the conscience of the party. The platforms get a lot of attention at this time, but they have a funny way of being virtually forgotten about as soon as the real campaigning begins in the presidential race.

The presidential race is different from the election for a leader in, say, Canada or Britain. That's because the United States has a presidential form of government. The Canadians and the British have a parliamentary system.

In the parliamentary system the voters vote for the party, not the person, of their choice. It is the party that has won the most seats in an election that dominates the next parliament. The leader of the winning party then becomes the next prime minister.

This is different from the American system, where voters can actually vote for a Republican president as a person, but at the same time elect Democrats to control both houses of Congress: the Senate and the House of Representatives. Thus while the president as the chief executive may well be the most powerful man in American politics, he may still have very little room to maneuver if the other political party controls the legislature, as the Congress is also called.

Another big difference between the American presidential system and the British parliamentary model is that in Britain, as in Canada, elections last only about three weeks.

Over the years both the Democrats and the Republicans have had a fair share of winners. And both major parties have had strong presidents and weak ones.