Who will run against Reagan? Here's how you can predict

President Ronald Reagan has done what everybody expected he would do. He has said he would run for a second term of office. How did he do it?

Simple. He just made a formal announcement that he would run again for President. Since he is already President and enjoys a high rating in the opinion polls, he will almost certainly be the choice of his own party, the Republican Party.

Nobody can say for certain who will be the Democratic Party candidate in the 1984 presidential election. This is because eight candidates so far have indicated that they will run for President.

Without question the front-runner among the Democratic hopefuls is former Vice-President Walter Mondale from Minnesota. Mr. Mondale was vice-president to President Jimmy Carter. His most serious challenger so far is Sen. John Glenn of Ohio, who first achieved fame as an astronaut. If you saw the space movie ''The Right Stuff'' you will remember that Astronaut John Glenn, played by an actor, had a prominent part.

After Mr. Mondale and Senator Glenn, perhaps the best known is former Sen. George McGovern of South Dakota. He lost overwhelmingly when he was the official Democratic candidate against Richard Nixon in 1972. Mr. McGovern's campaign manager then, Gary Hart, now is Sen. Gary Hart of Colorado, who this year is also in the field of Democratic candidates. The remaining four candidates are Sen. Alan Cranston of California, the Rev. Jesse Jackson of Illinois, Sen. Ernest F. Hollings of South Carolina, and former Gov. Reubin Askew of Florida.

These eight candidates, unless others join in later, will battle it out in a series of pre-election contests known as primaries. These primaries are held in many states. The first primary takes place at the end of this month, Feb. 28, in New Hampshire.

Although New Hampshire is one of the smallest states, it enjoys a political importance out of all proportion to its size. This is because, as the earliest presidential state primary, it gives a good and often very accurate guide to the way many people across the country feel about the candidates.

Since 1952, when New Hampshire began adding the names of presidential candidates to its primary ballots, no candidate who has won the presidency has lost the New Hampshire primary.

This suggests that whoever wins the Democratic primary in that New England state at the end of this month will most likely be the choice of the Democrats to stand against President Reagan in the actual presidential election. That takes place in November this year.

Primaries are not the only ways to measure a candidate's strength. There is a more informal method called the party caucus. Sometimes these affairs are so relaxed that party members may even vote for their choices in a neighbor's kitchen. These caucuses don't carry the same weight as the primaries, but they can't be discounted. The first caucus, in Iowa on Feb. 20, will actually be held before the nation's first primary.

When a nearly unknown candidate, Jimmy Carter, won the Iowa caucus as well as the New Hampshire primary, it was a clear signal that he stood an excellent chance of winning the Democratic presidential primary. He did.

So if you want to figure out who is most likely to be the candidate opposing President Reagan in November, watch for the results of the Iowa caucus and the New Hampshire primary.

If the same person wins in both of these two quite different parts of the country, there is an excellent chance he will be the ultimate victor, even though he must still travel many more miles and win many more primaries before he is elected president.