(Reuters) - Republican Donald Trump’s 2016 U.S. presidential candidacy has put the spotlight on the party’s selection process, with prominent Republicans trying to derail Trump’s efforts to sew up the nomination so they can force a showdown at the party’s July nominating convention.  To some, the prospect of a contested convention suggests the party elite will ride roughshod over the voters. But the emergence of state-by-state primaries and caucuses that let voters play a deciding role is a modern construct.  In 1952 Adlai Stevenson won the Democratic nomination without taking part in any primaries. In 1968, Hubert Humphrey became the Democratic nominee without participating in any primaries. At the heart of changes in both the Republican and Democratic parties since then is a desire to select a nominee who is likely to win in the general election. Following is a guide to how the nomination process has evolved.     Q: How did the nomination process evolve?      A: Political parties began to form early in the country’s existence, and party-based caucuses had developed by 1800. Caucuses in the 19th century were gatherings of the members of Congress of the various parties, and they picked the nominees.     Q: How did we get from nominees chosen by member of Congress to nominating contests decided by registered voters?     A: Over time the caucuses were criticized as failing to uphold the U.S. Constitution’s separation of powers because they made the selection of the president dependent on members of Congress. The election of 1812 brought that conflict to the fore because President James Madison was pressured by members of Congress to declare war on Great Britain if he wanted to be nominated for a second term. Q: How did that wind up changing things? A: The catalyst for change came in 1824 when the caucus process resulted in no candidate winning a majority of the votes in the Electoral College, a body of members selected by each state legislature to vote for president. The election ended up in the House of Representatives, which chose Secretary of State John Quincy Adams as president, even though Senator Andrew Jackson led both in the popular vote and in the Electoral College. That led to the two-party system, which has since dominated American politics, and to the nominating conventions.  Q: How were delegates to the conventions selected? A: Throughout the 19th century, the process was controlled by party insiders, who chose the delegates to the conventions. Those delegates voted for candidates in line with the wishes of party bosses. By the late 19th century there was a call to give individual voters a voice, and in the early 20th century some states began to hold primary elections to choose delegates for party nominating conventions.  The big change came in 1912 with the formation of the Progressive Party, also known as the Bull Moose Party, by former President Theodore Roosevelt and his supporters after Roosevelt failed to win the Republican nomination for a third term even though he won nine out of 10 primary elections held. The rest of the delegates at the party’s convention had been selected by party bosses, and they voted for William Howard Taft as the Republican nominee.  The Progressive Party declared itself dedicated to upholding the principles of “government of the people, by the people and for the people,” and it called for primary elections for party nominations, saying there should be “Nation-wide preferential primaries for candidates for the Presidency.” Q: Were those primaries the same as today’s nominating contests? A: Not really. As the 20th century progressed, more and more states began to hold primary elections to give citizens a voice. But primaries or caucuses weren’t held in all states and voters still didn’t control the nominating process. Primaries became a sort of bellwether, sending a message to the party leaders, who controlled most of the delegates, as to whether a candidate would be viable in the general election. In 1960, when John Kennedy became the Democratic nominee, his victory in the West Virginia primary was seen as a strong sign that a Catholic could win the votes of Protestants. Q: So even in the 1960s it doesn’t sound like primaries decided who the candidates would be. What changed? A: Let’s look at the Republican and Democratic parties separately, because the parties set the rules for choosing delegates for their nominating conventions. The Democrats have tinkered with their delegate selection system in response to almost every election since 1968, as they struggled with the push-and-pull of who should pick the Democratic nominee: the party insiders who believe they know who can ultimately win the presidential election or the voters themselves. Here’s the short-form history of what the Democrats have done: The 1968 election, when Hubert Humphrey, who eschewed the primaries in favor of courting officials who controlled the delegates, lost to Republican nominee Richard Nixon, led to reforms that essentially gave control of the nominating process to the popular vote. A number of states decided the easiest way to comply with the new rules would be to hold primaries. The 1972 election, when Democratic Senator George McGovern lost in a landslide to Nixon, spawned a sort of backlash, with a call to return some control over the nominating process to people who were expected to be party regulars and who would care more about party interests. State parties would be allowed to name up to 25 percent of a state’s delegates - a class now called “superdelegates.” The 1980 election, when President Jimmy Carter - who had battled Senator Ted Kennedy for the Democratic nomination - lost to Ronald Reagan, raised a call for more control by party leaders and public officials over the nominating process. The Democratic Party would now allow states to create winner-take-all processes for districts and mandate the election of unpledged party leaders and elected officials - the superdelegates. Those winner-take-all districts and the superdelegates became key to the selection of Walter Mondale as the Democratic nominee in 1984. He lost to Reagan. In 1988 the Democrats banned winner-take-all districts and said a candidate would have to win at least 15 percent of the popular vote to be awarded any delegates. Those rules still hold, though the number of superdelegates has grown. In 2008 superdelegates accounted for more than 18 percent of the total, and they ended up being the key deciders after neither of the top two Democratic contenders - Barack Obama and Hillary Clinton - won a majority of the pledged delegates. Q: Is that similar to the Republican situation? A: In a way, yes. The Republican Party does not have any superdelegates, but if no candidate wins a majority of the delegates prior to the July convention the delegates become free to change their votes, making them the deciders. Q: Has the Republican Party engaged in the kind of changes the Democrats have? A: No. The Republicans have given the states a lot of leeway in setting the rules for their primaries and caucuses. States can choose whether to award delegates proportionate to the popular vote, via a winner-take-all method or a combination of the two. The Republican Party in 2010 did say that states had to use proportional allocation for a certain period early in the season of nominating contests. That period was changed for the current election year to just the first two weeks of March. Q: So what happens if the Republicans end up with a brokered convention? A: A brokered convention would occur if no candidate has won a majority of the delegates ahead of the convention. (The magic number this year is 1,237 delegates.) At the convention, the delegates must initially vote according to the outcome of the popular vote from each state. But if no candidate has a majority of the delegates, that first ballot will be inconclusive. So the next step is the brokered convention, when the delegates are no longer bound by the outcome of the popular vote and can vote according to their own wishes. The vote is taken as many times as necessary until one candidate clears a majority of the delegates - something that occurs with a lot of dealmaking on the sidelines. 
How selecting U.S. presidential candidates became the people's choice