After a divisive primary challenge, U.S. Senator Bernie Sanders (I-VT) has taken to the campaign hustings urging voters to mark their election ballot for his former foe, Democratic Presidential nominee Hillary Clinton. Sanders tells voters Clinton is "the superior candidate" and that her Republican opponent, Donald Trump, is a "Pathological liar."

Sanders is following the precedent of most candidates who lose their party's Presidential nomination. It is commonplace for them to urge their supporters to support the party's nominee.

However, if the nominee is elected, there is history of the vanquished primary rival becoming a thorn in his/her side.

Sanders became a tribune of the progressive left. With the formidable support of over 13 million Democratic primary voters, Sanders has the electoral bone fides to lead the charge against Clinton should she pivot to the center once in office. He could become her biggest critic should she pursue an interventionist foreign policy, support trade deals opposed by labor and environmental groups, or become an ally of the large financial institutions which helped to fund her campaign.

There is a history of defeated primary foes becoming critics of the President who beat them. In 1892, former President Grover Cleveland mustered the Democratic Presidential nomination by defeating a former ally, U.S. Senator David B. Hill (D-NY). In another political lifetime, Hill had been a ticketmate with Cleveland. Cleveland was Governor and Hill was Lieutenant Governor of New York. With Cleveland's blessing, Hill was elected to succeed him.

However, the two developed irreconcilable differences, with Hill supporting bi-mettallism, where both gold and silver would be certified as legal tender, and economic nationalism, favoring a higher protective tariff. Contrariwise, Cleveland advocated hard money, supported the Gold Standard, and a lower protective tariff.

After losing the nomination, Hill got in line and endorsed his political friend turned foe. However, once Cleveland was elected, Senator Hill led the charge in blocking two of Cleveland's nominees to the U.S. Supreme Court.

Four years later, the Republicans selected former Ohio Governor William McKinley for President. McKinley got his political revenge by defeating Thomas Bracket Reed of Maine for the nomination. When McKinley was in the House, Reed had upended him for the post of U.S. House Speaker. While Reed supported McKinley in the General Election, he became indignant at the President for his support of the Spanish American War. Reed, a non-interventionist on foreign policy, gave up his gavel in protest, resigning from the Speakership and his House seat in the middle of his term.

In another example, New York Governor Al Smith was once an ally of Franklin D. Roosevelt. In fact, in 1924, when the Republicans nominated Roosevelt's cousin, Theodore Roosevelt Jr., to challenge Smith, Franklin was foursquare for Smith. That year, Roosevelt also nominated Smith for President at the Democratic National Convention, bestowing upon him the moniker: "The Happy Warrior."

However the ambitions of both men collided in 1932 as Roosevelt vanquished Smith for the nomination. Being a good soldier, Smith campaigned for Roosevelt in the General Election, singing his praises in the critically important state of Massachusetts. Smith, a Catholic, was wildly popular with the state's proliferating Catholic voting block.

However, Smith became a critic of Roosevelt. He came to see Roosevelt's "New Deal" as too pervasive. Smith lambasted Roosevelt for pitting "class against class." Smith even actively campaigned for Roosevelt's Republican opponents in 1936, and again in 1940, Alf Landon and Wendell Willkie, respectively.

More recently, in 1976, the Democrats nominated former Georgia Governor Jimmy Carter, who was elected as an outsider to the Washington establishment and who branded himself as "untainted" by politics. On the heels of the Watergate imbroglio, voters were looking for someone from outside of the Beltway establishment. Thus Carter defeated the early frontrunner and structural candidate, U.S. Senator Henry "Scoop" Jackson (D-WA.), who was first elected to the U.S. Congress in 1940.

Jackson however was a party man through and through. Accordingly, he hit the campaign trail hard for Carter.

Yet once Carter was elected, Jackson was a frequent critic of his foreign policy. Jackson, who favored a hard line on Russia, was a cheerleader for the opposition to the Second Strategic Arms Limitation Treaty negotiated by Carter and the Russians. Jackson led the effort against Carter's nomination of Paul Warnke as Arms Control Negotiator. Furthermore, in 1980, he endorsed Carter's primary opponent, U.S. Senator Ted Kennedy (D-MA.)

However, that praise was supplanted with condemnation when Clinton became President. Tsongas, a deficit hawk, believed that Clinton was spending too much money rather than promoting fiscal austerity. Tsongas told the New York Times that Clinton was a "direct threat to my children's generation" and that Clinton's "biggest problem is a lack of moral authority." In 1994, Tsongas even floated the idea of forming a third party which would nominate General Colin Powell to challenge Clinton for re-election.

Another rival of Clinton was U.S. Senator Bob Kerrey (D-NE). During the primary sweepstakes, Kerrey, a Medal of Honor winner, questioned why Clinton did not serve in Vietnam, opining: "There is an apparent unwillingness to accept responsibility for a decision that was apparently based on conscience . . . I quibble with a decision that was premeditatively designed to further a political career, even then." Kerrey also suggested Clinton "should not be the nominee of the party because he will not be able to win."

Still, Kerrey put that behind him and campaigned for Clinton in the General Election. He was even a contender to become Clinton's runningmate.

However, after Clinton became President, Kerrey called Clinton an usually good liar." Kerrey said of Clinton: "There's a tendency at times to play the victim-card a bit too much . . . It doesn't trouble me; I get angry with it.'' Kerrey and his Republican colleague John Danforth of Missouri promulgated a report calling for entitlement reform. However, Clinton rejected it. Kerrey even considered challenging Clinton for re-election, ultimately deciding against it.

Alternatively, some rivals have become allies of the President who defeated them. For example, in 1952 there was a bitter internecine feud between General Dwight D. Eisenhower and U.S. Senator Robert A. Taft (R-OH) for the GOP nomination. Eisenhower eked out a victory. However, that rivalry was set aside, as Taft, now the Senate Majority Leader, shepherded through Eisenhower's legislative agenda his first year in office. The two former rivals even became social friends. However, Taft died just over half way into Eisenhower's first year in office.

Similarly, in 1988, Vice President George H.W. Bush and U.S. Senate Majority Leader Bob Dole (R-KS) were embroiled in a political battle royale. At one point, the two candidates were on the air at the same time. Brokaw asked Bush if he had anything to say to Dole. Bush responded: "No, just wish him well and [we'll] meet again in the South." Brokaw then asked Dole if he had anything to say to Bush. Dole was inflamed by an advertisement the Bush campaign was running accusing him of "straddling on taxes." Dole responded: "Yeah, tell him to stop lying about my record."

However, Dole became Bush's chief defender in the Senate and was instrumental in supporting Bush on foreign and domestic affairs.

In the present election, Bernie Sanders has gone from political obscurity to a cult figure in progressive circles. Should Hillary be elected President, Sanders will likely be a constant presence, urging her to pursue a progressive agenda. In fact, Sanders could become her chief antagonist should she govern as a centrist. He could also be her chief exponent should she act as a progressive Democrat.

Today there appears to be a rapprochement between Sanders and Clinton. Should Clinton be elected President, she will need the support of Sanders. Sanders will clearly be in the political catbird seat, and could be a thorn in the side of Clinton should she fail to give proper deference to progressive polices.