When did the process of using popularly elected, pledged delegates to elect presidential nominees begin?
The delegate based national convention can be traced back to 1832. Up until the 1824 election, known by Andrew Jackson's supporters as the "Corrupt Bargain," presidential nominees were selected directly by elected party leaders in congress. The 1824 election, however, witnessed the rise of the populist Jackson, who won the popular vote, but lost the electoral college when Henry Clay decided to throw his electoral votes to John Quincy Adams in return for the Vice Presidency. After that election, Jackson toured the country, becoming the first presidential candidate to appeal directly to "regular American voters," instead of party bosses. By 1832, the major political parties no longer selected presidential nominees through congressional votes, but instead held national conventions, which were still dominated by party bosses, who selected delegates to carry out their will and choose nominees.
This top-down, party-boss centered selection of delegates kept the nominating process firmly in the hands of senators, congressman, governors, state legislatures and industry titans, who had immense sway over the selection of presidential nominees. So while the delegate system dominated the presidential nominating process by the mid-1800s, these were not pledged delegates, or delegates chosen by popular vote.
In fact, it was not until 1910 that Oregon became the first state to hold presidential primaries, in order to give the general electorate a voice in the nominating contests. Yet even by 1920, when twenty states had adopted the presidential primary system, the delegates chosen at the national conventions were still not "bound" by the will of the people of their state, who voted in primaries. The delegates to the national conventions remained free to vote their conscience, or just as often, to vote the way their party's bosses wanted.
This tension played out in the 1912 Republican National Convention when incumbent President William Howard Taft clinched the nomination despite the fact that former president, Teddy Roosevelt, had been the more popular candidate in the non-binding primaries. The selection of Taft led to a rupture in the Republican Party, and as a result, Roosevelt ran as an Independent. This split in the Republican ticket allowed the progressive Democrat, Woodrow Wilson, to become president.
Yet it was not until after the chaotic 1968 Democratic National Convention that the modern system of “pledged delegates” was born. The 1968 Democratic Convention erupted into a floor fight and riots on the streets of Chicago after supporters of the late RFK backed anti-Vietnam War candidate, Eugene McCarthy, only to have their party leaders chose the pro-war, vice president, Hubert Humphrey, as the party’s nominee, despite the fact that he had not won a single primary. Shortly thereafter, the Frasier-McGovern Commission, chaired by then senator and future Democratic presidential nominee, George McGovern, established the “binding delegate system” that exists today in recognizable form in both the Democratic and Republic parties. Under this system, party nominees are chosen (in large part) based on the outcomes of primary elections in individual states, making the selection of presidential nominees more democratic and representative of the country as a whole.