Electability is Joe Biden’s calling card. Without that perceived advantage, his viability as a presidential candidate crumbles.

Democrats want to beat President Trump more than anything. Nominating a candidate who can slay the dragon has immense allure. A Fox News poll finds that 94 percent of Democratic voters say ousting Trump is important to them.

What is electability? In a nomination fight, it’s a candidate’s capacity to win the general election. It’s a perception, and does not have to be accurate to have impact. If rank-and-file Democrats and high ranking party officials believe Biden has the best chance to topple Trump, right or wrong, that perception matters.

While electability can be a powerful force––it helped George W. Bush, John Kerry and John McCain win nominations––it’s ephemeral. Perceptions change as polls fluctuate and events unfold.

Electability cuts two ways in the current campaign.

One focuses on the likelihood that any particular Democrat will beat Trump. A poll from The Economist/YouGov finds that 65 percent of Democrats think Biden would “probably beat” the president. That compares to 63 percent for Elizabeth Warren and 56 percent for Bernie Sanders. The former vice-president’s edge is slight because primary voters believe, or at least hope, that multiple candidates could unseat Trump.

The other perception digs deeper––and focuses on which candidate has the best chance to beat Trump. It is on this measure that Biden shines. A Washington Post/ABC News poll finds that 42 percent of Democrats believe Biden has the best shot at beating Trump. Trailing is Warren at 17 percent and Sanders at 16 percent.

General election surveys continue to show Biden running best. Swing state polls recently conducted for the New York Times by Siena College amplify Biden’s advantage, especially over major competitor Warren. While Biden leads Trump by 2 points in decisive Florida, Warren loses by 4 points. While Biden beats Trump by 3 points in Pennsylvania, Warren runs even. While Biden ties Trump in Michigan, Warren falls short by 6 points.

Heavy reliance on using polls to prove electability, however, is risky business. A history lesson is instructive.

After months of indecision, New York Gov. Nelson Rockefeller jumped into the 1968 race for the Republican presidential nomination in late April of that tumultuous year. He hung his candidacy on the electability hook. “Rocky can win” was his slogan.

Polls showed that Rockefeller would be a stronger GOP candidate against Vice President Hubert Humphrey, the likely Democratic nominee, than would former Vice President Richard Nixon, the frontrunner for the Republican nod. Nixon had the image of a loser––having been defeated for president in 1960 and governor of California in 1962.

Seen as moderate and even liberal, Rockefeller did better with Democrats and independents than other Republican politicians of the era. While that didn’t help him with conservatives in his own party, it did help his standing in general election polling. Hence, his message to Republicans: Like me or not, I can win and Nixon can’t.

But a funny thing happened. A Gallup poll came out just days before the start of the Republican convention showing Nixon doing better than Rockefeller against Humphrey. Rocky was done––hoisted on his own petard.

Moral of the story: Hard won as it may be, the perception of electability can be fickle.

The Democratic cognoscenti see Biden, despite his weaknesses, as the strongest general election candidate. Few party leaders believe Sanders, a self-proclaimed socialist, can topple Trump. Skepticism of Warren’s electability has, at least temporarily, halted her once impressive momentum.

Fear that none of these Democrats will be able to whip Trump may draw multi-billionaire Michael Bloomberg, the former mayor of New York, into the contest. The rationale for his candidacy will surely include electability––Biden is too fragile, Sanders and Warren are too far left, Pete Buttigieg is too young, so why not nominate a competent winner?

Something similar could be said for the candidacy of former Massachusetts Gov. Deval Patrick. If he’s not seen as electable, don’t expect him to get very far.

Electability is surely a factor this year, but polls alone may not be enough to prove it. Candidate appeal, skill and vision can turn electability into a self-fulfilling perception.

Biden’s campaign should wait a while before it prints “Joe can win” posters.

This column was published in the New Orleans Times-Picayune/The Advocate papers.