Could it happen?

OF course ''it'' can't happen in 1984, but then things are a little different on the American political scene, now that some unexpected voters turned up in Ohio and Indiana and put Gary Hart back in the running.

''It'' in this case is the kind of political convention that used to happen back in the old days before the presidential preference campaigns really got going and created the six-month-long tedium of the battle through the primaries, which, as intended by the reformers who created it, have largely taken away from party leaders the choice of presidential candidates.

The actual national conventions really mattered in those almost forgotten days, and the issue was sometimes long in coming. In 1912, for example, the Democrats took 46 ballots before they could finally agree to nominate Woodrow Wilson as their standard-bearer.

The Republicans that year renominated William Howard Taft on the first ballot because the convention was effectively controlled by the White House organization. But this outraged the rank and file of delegates, who preferred Theodore Roosevelt. ''Teddy'' pulled his delegates away, organized a new Progressive Party, held another convention, and handed the election to the Democrats and Woodrow Wilson.

Then in 1924 the Democrats had an even longer convention. It was deadlocked between William Gibbs McAdoo, the ''dry'' candidate, and Al Smith, governor of New York, who openly favored repeal of Prohibition, hence was known as the ''wet'' candidate. The deadlock lasted for 102 ballots. At the end the Democrats finally gave up and compromised on John W. Davis, a corporation lawyer. The Republicans won the the election of Calvin Coolidge.

The memory of those long, dreary strings of ballots, and the consequences, provided one reason for the proliferation of preferential primaries during following years. The idea was to give the voters a larger share in the selection of presidential candidates.

Well. We Americans did proliferate primary elections. And we did thereby stretch out of the selection process. And, perhaps, more voters have more say in the selection of the candidates, although there is a question whether the process is more reflective of the preferences of the majority. The system, as would be true of any system, is open to abuse. Too often, too few voters turn up at the primary polls to make the results accurately reflective.

But it should, and usually does, seem to avoid uncertainties when the actual conventions occur. Events were moving this year to two first-ballot nominations: Ronald Reagan by the Republicans and Walter Mondale by the Democrats.

But as often happens when something unexpected taps the political kaleidoscope, it suddenly takes on a new pattern. One can assume that Mr. Reagan will get the Republican nomination on the first ballot. But what about the Democrats? There is now a visible chance that Mr. Mondale will not get a first-ballot nomination. If the delegates committed to Gary Hart and Jesse Jackson, and the uncommitted delegates, should pool their resources, they might be able to head off a quick Mondale victory. If so, then what happens?

The Democratic convention could at that point be blown wide open. Almost anything could happen. Suppose a lot of delegates are by that time disenchanted with the three existing candidates and start looking around for a possible alternative. One already hears talk of Sen. Dale Bumpers of Arkansas or Gov. Mario Cuomo of New York. Both are mentioned as possible running mates for Walter Mondale, but also as possible alternatives for the top of the ticket.

Then Richard Nixon in a talk to the American Society of Newspaper Editors last week predicted a Reagan victory on election day - ''unless the economy goes down.''

That sent a shudder through the White House. It coincided with an upward drift in interest rates. Upward-drifting interest rates could slow the economy, push up unemployment, and push down the stock market. So it's a mystery story with an uncertain conclusion.