Larry Sabato of the University of Virginia devotes a Politico article to the conventional wisdom that former secretary of state, U.S. senator, and first lady Hillary Clinton is a lock for the 2016 Democratic presidential nomination.

Could Hillary Clinton be the next Richard Nixon? Now that’s a provocative question, but it isn’t quite what you think. …

… The Clintons are nothing if not shrewd, and they’ve lived through the entire era of postwar American politics. So Hillary Clinton would be the last to believe what I have heard with increasing frequency: that, in the end, no one of real heft, even Vice President Joe Biden, will challenge her for the Democratic nomination she nearly won in 2008, and she will steamroll over the minor contenders who do. Most frequently mentioned in the “minor” category are former Gov. Brian Schweitzer of Montana and Gov. Martin O’Malley of Maryland. (O’Malley also made a little-noticed appearance at the McAuliffe inauguration.) Sen. Elizabeth Warren of Massachusetts would be a major opponent should she run, but she insists she will not. When California Gov. Jerry Brown also bowed out, NBC News’s First Read called it “a reminder that Hillary Clinton will probably face little to no serious competition if she runs.”

Possible? Sure. But history’s guide tells us otherwise. A consensus choice for a major-party presidential nomination is exceedingly rare—and this is where the Nixon comparison comes in.

Incumbent presidents often find their second-term nominations nearly unopposed, though even in this rarefied group, there are notable exceptions: Harry Truman, Lyndon Johnson, Jimmy Carter and George H.W. Bush. (Truman and Johnson withdrew in 1952 and 1968, respectively, partly because of intra-party opposition.)

But when no incumbent was running, the only precedent for a consensus choice in the entire post-World War II era is Richard Nixon in 1960. This impressive feat was nonetheless achieved with some difficulty and embarrassment.