In July 1864, as President Abraham Lincoln prepared to run for a second term against General George B. McClellan, The New York Times editorialized: “We have had many important elections, but never one so important as that now approaching....The republic is approaching what is to be one of the most important elections in its history.” The Civil War had been raging for three years and seemed to be at a stalemate. Lincoln was for fighting on until victory, regardless of the cost. McClellan supported compromise and negotiation to end the bloodiest conflict in American history. As everybody knows, Lincoln won the election, the Union soon won the war, and McClellan’s reputation never recovered.Historians can argue about whether a McClellan victory would have actually changed the trajectory of the war. By November, 1864 Atlanta and Mobile had fallen. Sherman was on the march to Savannah. Sheridan had cleared out the Shenandoah Valley. Lincoln with his "blind memorandum" expressed his determination to carry forward with prosecuting the war even if McClellan should win. If McClellan had been sworn in as president on March 4, 1865 it is highly doubtful that, with Grant's army poised to break through around Petersburg, a President McClellan would have stepped back from that edge of victory. And would McClellan who was totally uninterested in slave emancipation been any worse than Andrew Johnson in his relations with the Radicals in Congress in administering Reconstruction? So perhaps the 1864 election wasn't as important a moment in our nation's history as it seemed back then.
The point is that we only know in retrospect if an election was such an important turning point. I'd argue for 1860 being the most important election since without a Lincoln victory it is not clear that the South would have seceded at that point. But maybe they would have done it a few years later. Perhaps war would have been pushed back long enough for medicinal knowledge of antiseptics and the connection between germs and the spread of disease to have saved thousands of lives that were lost in the actual Civil War. Given that Joseph Lister and Louis Pasteur were doing their research in the 1860s, imagine how the medical history of the Civil War might have been different if it had taken place 15 years later. Though lives saved on the battlefield through the use of antiseptics would have been small consolation to slaves waiting for emancipation.
The point is that labels such as "most important election" are meaningless at the time. We lack the historical perspective to see how the country and world changes as a result of that election. And the phrases has been used so often that it has become just a cliché. What they're actually telling you is that they really, really want you to vote for their guy.