WASHINGTON (AP) — It was 1948 when New York Gov. Thomas E. Dewey won the Republican presidential nomination on the third ballot of a contested convention in Philadelphia. Months later, he lost the general election to Democratic incumbent Harry S. Truman.

Could a real contested convention happen again, 68 years later?

If GOP front-runner Donald Trump stumbles in Tuesday's contests, there's a chance of a protracted fight in Cleveland in July should favorite sons Gov. John Kasich of Ohio and Sen. Marco Rubio of Florida defy the polls and take their respective states' winner-take-all primaries.

Delegates are generally bound to a candidate, at least on the first ballot. After that, watch out.

A look at the basics:

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HOW IS THE REPUBLICAN NOMINEE CHOSEN?

Whoever wins a majority of the 2,472 convention delegates, or 1,237 votes, at the Republican National Convention in Cleveland is the party's nominee.

What happens if there's no winner on the first ballot? Delegates vote again and again until one candidate gets a majority. On subsequent ballots, most delegates are free to vote for whomever they please.

Remember, too, that most delegates, roughly 4 out of 5, are selected at state or district party conventions and are commonly party activists and loyalists — the establishment types who are more likely to be aghast at the prospect of a Trump nomination.

Relatively few states directly elect delegates already pledged to a candidate. In many cases, those state and local conventions are going to choose actual delegates who may not have a real allegiance to Trump, though the vast majority of such delegates are bound on the first ballot to vote based on the results of their state's primary or caucus.

So if Trump cannot win on the first ballot, the logical next question is how strongly his delegates support him.

The anybody-but-Trump crowd hopes that denying Trump a majority on the first ballot could rally a majority of the convention around another candidate — maybes Texas Sen. Ted Cruz or even someone not running, like House Speaker Paul Ryan of Wisconsin.

How many votes Trump gets on the first ballot — whether he falls just shy of 1,237 or significantly short of it — probably will factor into the dynamics of what would happen on a second ballot.

At a GOP debate on Thursday in Miami, Trump said that "whoever gets the most delegates should win." Cruz, who might get stepped over in a contested scenario, said a contested convention would be "an absolute disaster."

"There are some in Washington who are having fevered dreams of a brokered convention," Cruz said. "They are unhappy with how the people are voting and they want to parachute in their favored Washington candidate to be the nominee. We need to respect the will of the voters."

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IS SECURING THE NOMINATION FAIRLY SIMPLE?

No, it gets tricky.

To have one's name placed in nomination, a candidate needs to win the support of a majority of delegates of at least eight states. Trump is the only one who seems sure to qualify under the rule, which was orchestrated in 2012 by Mitt Romney supporters to deny then Texas Rep. Ron Paul an opportunity to be nominated and garner attention at that year's convention.

What might happen on a second ballot? Would anyone else qualify under the eight-state rule? If not, it could be difficult to deny Trump the nomination on a second ballot if he is close to the threshold and is the only candidate able to be nominated.

The rules favor Trump, as they are now written. Cruz might qualify under the eight-state rule, but it's not a sure bet. The other candidates look unlikely to meet this test.

All the rules, however, are subject to change by the convention's Rules Committee, which is generally stocked with party insiders. Any proposal to change the rules requires a majority vote of the convention, and any effort to change the rules to make it easier to defeat Trump is sure to spark a floor fight with passionate Trump supporters that could turn ugly.

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WHAT ELSE?

In 1976, a real battle loomed between President Gerald Ford and challenger Ronald Reagan, but Ford won on the first ballot. Since then, GOP conventions have been little more than coronations, so there's the question of how well the system would handle anything more than that.

Basketball arenas aren't the ideal place for potentially complex parliamentary maneuvering. The convention's chairman, Speaker Ryan, has promised to be an honest broker, but he also has discretion in managing the floor dynamics if an effort to defeat Trump gets under way.

There are competing interpretations about the present set of rules. If a candidate needs backing from eight states to be placed in nomination, does that include territories such as Guam and the Northern Mariana Islands, which could help Cruz?

A quirk in the rules from 2012 could even make it theoretically possible to elect a vice presidential nominee loyal to Trump before a presidential nominee is chosen.