The conventional wisdom following Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE's eyelash victory over Sen. Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE (I-Vt.) in the Iowa Democratic caucuses and a potentially favorable contest looming for Sanders in New Hampshire is that Democrats are likely to endure a marathon primary struggle. If so, the near-certain outcome is a Republican victory in the fall campaign. Not since the election of Republican James Garfield in 1880 has the party in power emerged victorious in November after a serious nomination contest.

ADVERTISEMENT

One person can forestall this unfortunate scenario for the Democrats: President Obama. So far, the usually shrewd Obama is making the greatest political mistake of his career by not taking control of his party's presidential nomination and endorsing Clinton, the only candidate capable of uniting the party and avoiding a ruinous nomination battle. This mistake may cost the president most of his legacy, which cannot withstand a victory this year by a Republican presidential candidate.

With presidential coattails likely assuring continued GOP majorities in Congress, Republicans would repeal the Affordable Care Act. They would weaken controls over Wall Street and repudiate both the Iran nuclear agreement and the Paris accords on combating climate change.

A Republican president would scrap Obama's pathbreaking executive orders and actions on immigration, the environment, gun control and the minimum wage. He would likely appoint at least two Supreme Court justices and set the course of American jurisprudence for a generation. President John Adams served four years in office and his Federalist Party subsequently unraveled. Yet his appointee John Marshall reigned as chief justice for more than 30 years and put the imprint of Federalist principles firmly on the U.S. Constitution.

In August, White House Press Secretary Josh Earnest said that "The president certainly believes that a spirited contest would be in the best interests of our party." Perhaps Obama was thinking of his own contest with Clinton that led to his nomination and election in 2008. But therein lies Obama's critical mistake: In 2008, Obama represented the challenging party, not the party holding the White House.

A spirited nomination contest may be healthy for the challenging party, but it portends near-certain defeat for the party holding the White House. Voters have almost invariably declined to accord the incumbent party another four years in office when it cannot agree on its leadership and policy direction.

Not just Obama in 2008, but many other rigorously contested challenging party candidates have won decisive general election victories. The three most prolific challenging party candidates in modern American history — Republican Warren Harding in 1920 (60 percent of the popular vote), Democrat Franklin Roosevelt in 1932 (57 percent) and Republican Dwight Eisenhower in 1952 (55 percent) all trod a torturous path to their party's presidential nomination.

Dark-horse candidate Harding won a ninth-ballot nomination after the front-runner deadlocked in earlier tallies. Roosevelt overcome conservative Democratic opposition on the third convention ballot only by offering Texas Rep. John Nance Garner the vice presidential nomination in return for support from the Texas delegation. Eisenhower and Ohio Sen. Robert Taft fought to a draw in the pre-convention campaign. Eisenhower won the nomination only after the convention decided a decisive credentials dispute in his favor.

No such positive results have followed bruising battles for the incumbent party nomination. Since Garfield eked out a general election victory by one-tenth of 1 percent of the popular vote in 1880, the incumbent party has endured major nomination struggles in eight presidential elections, losing every time in November. Defeated candidates include Democrat William Jennings Bryan in 1896, Republican William Howard Taft in 1912, Democrat James Cox in 1920, Democrat Adlai Stevenson in 1952, Republican Gerald Ford in 1976, Democrat Hubert Humphrey in 1968, Democrat Jimmy Carter in 1980 and Republican John McCain in 2008.

In contrast, in only three of 25 contests (12 percent) since 1880 has a relatively uncontested nominee lost the popular vote: Republican Herbert Hoover in 1932, Republican Richard Nixon in 1960 and Republican George H. W. Bush in 1992. In 1888 and 2000, two consensus nominees, Democrat Grover Cleveland and Democrat Al Gore, won the popular vote but lost in the Electoral College.

The Democrats in 2016 need not follow a nearly certain pathway to defeat. Obama still has the opportunity to rectify his mistake, take charge of his party's nomination and get the Democrats on the winning side of history.

Lichtman is distinguished professor of history at American University in Washington and co-founder of The Keys to the White House, a prediction system that has successfully forecast the popular vote winner of all presidential elections since 1984. His latest book, "FDR and the Jews" (Belknap, 2013), co-authored with Richard Breitman, won the National Jewish Book Award in American Jewish History and was a finalist for the Los Angeles Times Book Prize in History.