Political parties are at a comparative advantage when they unify behind their Presidential nominee. Predictably, supporters of the presumptive Democratic nominee Hillary Clinton will do everything possible to heal the chasm with the progressive left: those who supported her primary opponent U.S. Senator Bernie Sanders (D-VT). Their fear is, assuming Hillary pockets the nomination at the Democratic National Convention in July, Sanders supporters will not support Hillary in the General Election, enabling Republican Donald Trump to win the Presidency. To propitiate Sanders' supporters, Democrats will likely incorporate part of his message into the party platform and grant him a primetime speaking slot at the Convention.
In 1880, the Republican Party was split asunder between supporters of Civil Service Reform and those who supported the status quo. The party nominated U.S. Representative James Garfield (R-OH) for President. Garfield was a vociferous supporter of Civil Service Reform. To unify the two competing factions, the party nominated Chester A. Arthur, a hero to opponents of Civil Service Reform. Arthur had been fired from his position as Collector of Customs for the Port of New York by Republican President Rutherford B. Hayes for refusing a Presidential directive to disallow his employees from concomitantly working as party functionaries. The integrated ticket of Garfield and Arthur managed to eke out a narrow victory.
Contrariwise, in 1896, with the country mired in an economic depression, the Democratic Party nominated William Jennings Bryan, a populist who heralded government intervention in the economy, favored implementing a graduated income tax, and favored bimetallism (allowing both gold and silver to be certified as legal tender). These progressive policies were in direct contrast to Democratic President Grover Cleveland's policies. Cleveland opposed all three proposals and preached the gospel of fiscal austerity even during such economic peril.
Bryan's recreance to Democratic orthodoxy was too much for establishment Democrats, including Cleveland. Inflamed by Bryan's nomination, they hastily formed the National Democratic Party and nominated U.S. Senator John M. Palmer of Illinois, who held to the party's conservative ideals. The party's platform supported: "sound money" and was "opposed to paternalism in class legislation."
The party schism contributed to Republican William McKinley winning the Presidency that year. While the National Democratic Party soon dissolved, an internecine bloodbath continued for the next three elections between the liberal and conservative factions of the party. The shattered party lost the next three Presidential elections. Bryan won the nomination two more times, in 1900 and 1908. When the party chose the conservative Clevelandite Alton B. Parker in 1904, Bryan did not endorse him, averring: "No self-respecting Democrat would vote for him."
More recently, in 1964 the Republican Party nominated the insurrectionist conservative U.S. Senator Barry Goldwater (R-AZ) to the chagrin of the more moderate Republican establishment. Goldwater made little effort to mitigate his Conservative message, telling the American people at the party's National Convention: "Let me remind you that extremism in the Defense of Liberty is no vice. And let me remind you also that Moderation in the pursuit of justice is no virtue." Goldwater had horrified the Republican establishment by suggesting making Social Security voluntary. In addition, Goldwater embarrassed the party high command when he quipped in a discussion about the precision of nuclear missiles: "I don't want to hit the moon. I want to lob one into the men's room of the Kremlin and make sure I hit it."
Consequently, Goldwater's moderate opponents for the nomination (New York Governor Nelson Rockefeller and Pennsylvania Governor William Scranton) did not endorse him in the General Election contest. In addition, Republican Governor George Romney of Michigan, a rising star in the GOP, refused to back Goldwater. In fact, his re-election campaign mailed out about 200,000 mock ballots showing Wolverine state voters how to mark their ballots for Democratic President Lyndon B. Johnson for President and Romney for Governor. Failing to garner much support outside of the conservative base, Goldwater won just six states in the General Election.
Four year later, in 1968, it was the Democrats who failed to harmonize. President Lyndon B. Johnson chose not to seek re-nomination after nearly losing the New Hampshire Primary to U.S. Senator Eugene McCarthy (D-MN). The party became bitterly divided chiefly over the President's escalation of troops in Vietnam. Old-line establishment Democrats supported the President, while the "new left" demanded an end to U.S. involvement in the war.
After Johnson dropped out of the race, Vice President Hubert Humphrey jumped into the Democratic Presidential sweepstakes. Humphrey was not popular with the new left because of his support of the Johnson administration's Vietnam policy. While the McCarthy brigades worked to secure delegates in the Democratic primaries, Humphrey only participated in one primary, South Dakota, which he lost. His campaign dispatched favorite son candidates like Governor Roger D. Branigin of Indiana to appear on the ballot in Humphrey's place. These candidates then released their delegates to Humphrey at the Democratic Convention.
In addition, Humphrey collected the support of delegates in those states which did not hold primaries. In these states the party elite controlled the delegates. As a result of this somewhat undemocratic process, riots ensued in front of the Democratic National Convention in Chicago, and anti-war liberals embarrassed Humphrey on the campaign trail, sometimes heckling him at his rallies.
Humphrey was able to bring some McCarthy supporters into his fold late in the campaign by announcing that as President he would unilaterally halt the bombing of North Vietnam "as an acceptable risk for peace." However, McCarthy did not endorse Humphrey until about a week before the election, and he did so in a very snide way, asserting to his devotees: "I'm voting for Humphrey and I think you ought to suffer with me."Humphrey made the best of this tepid endorsement, declaring it made him a "happy man." McCarthy's luke warm support for Humphrey coupled with Humphrey's failure to coalesce the support of the party base around his candidacy in the General Election is blamed by some in the Humphrey camp for giving the election to Republican President Richard M. Nixon.
To win a Presidential election, the nominee must unify all significant bloodlines of his/her political party. With the increasing ideological homogeneity of political parties, that task is easier today than it was in the past. For most of the Twentieth Century, both parties encompassed liberal, moderate, and conservative bloodlines. Conservative Southern Democrats often found little in common with Frostbelt liberals. Eastern liberal Republicans were rarely in concert with Western Libertarians. Today however, the Democrats are clearly defined as a center-left party while the Republicans are clearly defined as a center-right party.
From an ideological perspective, the differences between Sanders and Clinton are de minimus compared to the differences between Bryan and Cleveland in the Democratic Party in 1896 or the GOP establishment and Goldwater in 1964.
There are some Sanders supporters who will never support Hillary. They believe Sanders represents a revolution, and they cannot reconcile Hillary's support for military interventions, accepting and soliciting financial donations from Wall Street, or past support for Free Trade Agreements. However, these supporters are in a minority. In fact, a recent poll shows that 86% of Democratic primary voters will support Hillary in the General Election. The Hillary camp will likely role out the red carpet for Sanders and genuflect to his supporters to get them to support Hillary in the General Election.