As Tuesday’s second presidential debate approaches, Democrats worried by President Obama’s poor performance in the first debate have taken comfort in a false historical comparison: Both George W. Bush and Ronald Reagan, we are told, lost their first presidential debates but went on to win reelection. That may be a consoling storyline, but it is highly misleading. Here is why:

Contrary to popular myth, John Kerry did not win his first debate with George W. Bush. While the pundits declared Kerry the victor, the truth is he lost — because he was the only candidate on stage who committed a major gaffe that haunted him till Election Day. It was during that first debate that Kerry stumbled and declared that America must pass a “global test” before taking preemptive military action to protect the country. Kerry’s “global test” quickly became fodder for late-night comics, a staple of Bush’s campaign stump speech, and the subject of a devastating political ad. It’s hard to argue that Kerry “won” that debate when he handed his opponent ammunition to use against him in the closing weeks of the campaign. By contrast, Mitt Romney made no gaffes in his first debate with President Obama that undermined his otherwise commanding performance. In other words (to paraphrase a line from another famous debate), he was no John Kerry.

As for the first Reagan-Mondale debate in 1984, Mondale did not beat Reagan so much as Reagan lost. He looked tired and at times confused, which fed the perception that he was too old for a second term. But Reagan had one major advantage going into the second debate that Obama does not enjoy today: In 1984, it really was “morning in America again.” A majority of the electorate believed that the country was headed in the right direction and credited Reagan with the turnaround. Today, by contrast, just 38 percent of Americans say the country is heading in the right direction, while 60 percent think it’s on the wrong track.

Moreover, Reagan recovered in the next debate by delivering one of the most devastating lines in the annals of presidential debate history, when he said “I am not going to exploit, for political purposes, my opponent’s youth and inexperience.” Such moments are exceedingly rare in presidential debates. It is not unimaginable that Obama could match that Reagan moment on Tuesday, but it is an extremely high bar for him to clear.

Obama’s advantage going into the debates was that, while Americans were deeply dissatisfied with the direction of the country, they had serious concerns about Mitt Romney’s fitness for office — thanks to the $100 million barrage of negative ads Obama had unleashed painting Romney as a reckless reactionary and a heartless corporate raider. But Romney’s debate performance dismantled that impression in 90 minutes — and freed many persuadable voters to go with him. The polls have been cutting his way ever since.

This means that in the next debates Obama can’t simply draw even or win by split decision. He needs a clear and decisive victory — one that will not simply slow Romney’s momentum but reverse it. Romney, by contrast, does not need a knockout moment. He just needs to turn in another steady, confident and even-heeled performance. In the next two contests, a tie goes to the challenger.

This is not to suggest by a long shot that Romney has the debates wrapped up. He could still “pull a John Kerry.” After all, Romney is the candidate who gave us the 47 percent, “my wife drives a couple of Cadillacs,” and “I’m not concerned about the very poor.” But Romney’s performance in the first debate was so disciplined, it seems doubtful he would blurt out something unscripted.

In the end, the biggest challenge Obama faces is one that is not in his control: While the president can do better, he can’t make Romney do worse. In Denver, Romney put in one what may well be the strongest performance in the age of televised presidential debates. If the same Romney shows up Tuesday, even a near-perfect debate from Obama might not be enough to change the momentum of this race.

Marc A. Thiessen, a fellow with the American Enterprise Institute, writes a weekly online column for The Post.