In recent weeks, former Vice President Joe Biden's team has doubled down and explicitly appealed to electability and framing Biden as the sole candidate who can feasibly beat President Donald Trump.

Luckily for Biden, seniors and African-American voters — two core constituencies in the Democratic base — prioritize defeating Trump above all else.

"Your candidate might be better on healthcare than Joe is, but you have to look at who is going to win this election," his wife Jill Biden said last weekend, citing general election match-up polls showing Biden beating Trump.

There's just one problem: hypothetical general election polls haven't historically been a very good predictor of who actually wins elections.

At this point in the 2016 election cycle — late August 2015 — head-to-head matchups showed, on average, Clinton beating Trump 50% to 39%.

And in late 1991, Bill Clinton was 21 percentage points behind George H.W. Bush in general election match-ups but ended up defeating Bush by 5.6% in the popular vote.

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Former Vice President Joe Biden is pitching his campaign for president on a single, resounding message: beating President Donald Trump, and restoring the kind of stability Americans experienced under the Obama administration.

But in recent weeks, Biden's team has doubled down and explicitly appealed to electability, framing Biden as the sole candidate who can feasibly beat Trump, as Biden's wife former Second Lady Jill Biden did at a recent event.

On the subject of healthcare, Jill didn't directly argue that Biden's healthcare plan to strengthen the Affordable Care Act and introduce a public option to compete with private insurance was a better idea on its merits than the type of government-run, Medicare for All healthcare system other candidates are proposing.

Read more: Joe Biden's former brain surgeon explains why he's not too old to be president

Instead, she acknowledged that even if Biden's plan isn't the best one and voters don't like it, they should hold their noses and vote for him anyway.

"Your candidate might be better on healthcare than Joe is, but you have to look at who is going to win this election, and maybe you have to swallow a little bit and say 'I personally like so-and-so better,' but your bottom line has to be that we have to beat Trump," she said.

And in Biden's first 60-second TV ad spot airing in Iowa this week, Biden explicitly appealed to his own electability, citing four hypothetical general election polls from this summer that show him defeating Trump by margins of nine to 13 percentage points.

Biden campaign's message is working because voters care about electability

Biden's presidential bid hasn't been all smooth sailing so far. Not only have his progressive rivals ripped apart policy stances on everything from healthcare to climate change and immigration as too moderate and incremental, but he's faced scrutiny over his long and controversial record in politics — especially his record on issues related to race.

The former vice president has also committed a series of cringeworthy gaffes on the trail, was criticized for being too touchy with women and girls, and found himself in hot water for fondly recalling the days of "civility" in the Senate when he maintained close relationships with notoriously pro-segregation Senators.

Throughout his missteps, however, Biden has not only remained the frontrunner but has stayed steady or improved his performance in Democratic primary polls — especially among voters who prize electability.

"Joe Biden isn't the frontrunner because he's first in the polls, it's because of who he's polling well with," Ford O'Connell, a veteran GOP campaign strategist and adjunct professor at George Washington University's Graduate School of Political Management, previously told Insider.

"Biden has the majority of support with African Americans, particularly African-American women and seniors. The reason why that matters is because those two groups traditionally turn out the most in Democratic primaries," he added.

Read more: Joe Biden has been a self-professed gaffe machine for decades but Democratic primary voters don't seem to care, yet

Older voters in particular, who turn out to vote at higher rates than younger ones, prioritize defeating Trump most of all.

In a Gallup poll from late June, 67% of voters aged 50-64 and 71% of voters 65+ said that they prioritized nominating the candidate most likely to beat Trump over the candidate with whom they agreed the most on policy, compared to just 43% of voters aged 18-29.

And luckily for Biden, Democratic primary voters — especially seniors and black voters — consistently rank him as the candidate with the highest chance of beating Trump. But he is at risk of losing his status as the most electable candidate soon.

As Bloomberg recently reported, the percentage of Democrats who think Biden could beat Trump in Economist/YouGov polling has remained steady all summer at 65%.

But as Sen. Elizabeth Warren has surged in Democratic primary polls, the percentage of respondents who believe Warren could defeat Trump has increased 14 percentage points from 43% to 57% in the same timeframe.

"What is it going to take for Joe Biden to lose the nomination? It's very simple. Either the Democratic electorate has to believe that he can't go toe-to-toe with Donald Trump or someone else has to prove that they can. Until that happens, he's going to be the nominee," O'Connell told Insider.

Candidates former Vice President Joe Biden and U.S. Senator Kamala Harris. REUTERS/Rebecca Cook

There's just one problem — Biden might not actually be the most electable candidate

Democratic strategist Jess McIntosh, the former communications outreach director for Clinton's 2016 campaign, argued on CNN Tuesday afternoon that Jill Biden's comments on Biden's electability were misguided "because we are terrible at predicting who is electable."

McIntosh pointed out that when it came to previous presidential nominees like Bill Clinton, Mitt Romney, Barack Obama, Hillary Clinton, and Donald Trump, the conventional wisdom was "wrong on every single one."

Former Presidents Bill Clinton and Barack Obama were largely unknown on the national political scene and started out as underdogs at the back of the pack when they began their presidential campaigns, but both won two terms each by inspiring Democratic voters with unique and compelling campaign messages.

Read more: Obama told Biden advisers not to let the former Veep 'damage his legacy' in his 2020 presidential run

And in 2016, Republican primary voters similarly rejected the conventional wisdom that Marco Rubio or Ted Cruz would be the safest and most electable choices, and went with their gut to nominate Trump, someone who had never held elected office, held positions that broke with the conservative orthodoxy, and had a controversial and scandalous personal history.

As political scientist Seth Masket pointed out, hypothetical Quinnipiac University general election polls from February 2016 showed Democratic nominee Hillary Clinton handily defeating Trump by five percentage points, tied with Sen. Ted Cruz, and losing to Sen. Marco Rubio by seven percentage points. But Republicans nominated Trump anyway, and won.

"Everyone who was supposed to win lost, and everyone who was supposed to lose won," McIntosh argued on CNN, adding that, "telling someone to ignore their gut and vote for the candidate who will win instead of the one they like is telling them to throw the primary, because we have no idea who can win."

Head-to-head general election polls have historically been a bad predictor of the actual outcome this far out from the election

In her argument for why Democratic primary voters should pick Biden over other candidates, Jill Biden cited the head-to-polling showing Biden leading Trump as the main reason undecided primary voters should back him over other candidates.

"You've got to look at the polls ... and if they're consistent and they're consistently saying the same thing, I think you can't dismiss that ... if your goal is to beat Donald Trump, we have to have someone who can beat him," she said.

Not only has the conventional wisdom on electability been wrong many times before, but hypothetical general election polls, like the ones Biden's camp is relying on, haven't historically been a very good predictor of who actually wins elections, as FiveThirtyEight's Perry Bacon Jr. pointed out in June.

Read more: Nearly one-third of Democratic primary voters fear their party could blow it in 2020 if they're not moderate enough. Nearly the same amount of Democrats fear they'll lose if they're not progressive enough

At this point in the 2016 election — late August 2015 — head-to-head matchups showed, on average, Clinton beating Trump 50% to 39%, a margin of victory of 11 percentage points. In the end, Clinton defeated Trump 48% to 46% in the popular vote and lost the electoral college.

In 2016, FiveThirtyEight also conducted a broader analysis of general election matchup polls from every presidential election from 1944 to 2012. They found head-to-head matchup polls conducted a year before presidential elections were, on average, 11 percentage points off the final result.

For example, former President Bill Clinton hardly seemed electable in the later months of 1991 when he was 21 percentage points behind George H.W. Bush in the polls, but ended up beating Bush by 5.6% in the popular vote after the US went into a recession.

"Aside from the logic, telling us to settle now is way too early," McIntosh added. "There is no need to settle."