A McConnell defeat would be all but unprecedented.

Mr. McConnell is not only an incumbent senator who represents the party opposed to the White House in a midterm election, but he also comes from a state that opposes the president. Since 1956, only seven senators in these circumstances have lost re-election. The last time was in 1998, when John Edwards defeated Senator Lauch Faircloth in North Carolina.

But 2014 is not 1998. Back then, President Bill Clinton’s approval rating was in the mid-60s. Today, President Obama’s ratings are mired in the low 40s. Kentucky is also not North Carolina, which only narrowly voted against Mr. Clinton. Kentucky is an extremely favorable state for a Republican candidate: It voted for Romney and McCain by an average of 25 points to the right of the country.

From that perspective, there is no precedent for a McConnell defeat. No senator has lost in a state as favorable as Kentucky when the president represents the other party.