RFK Jr. can't win. But he and Cornel West could put Trump back in the White House.

A recent series of six swing-state presidential polls offered Democrats and President Joe Biden a welcome dose of optimism. The head-to-head matchup with former President Donald Trump had Biden winning Wisconsin by a point and tied in Michigan and Pennsylvania.

With the usual warning about the accuracy of polls this far out, a gentle breeze in the blue direction would give the president all three states and enough Electoral College votes to win reelection.

But the same Bloomberg/Morning Consult poll contained a dire warning. When minor party candidates like Robert F. Kennedy Jr., Jill Stein and Cornel West were added to the mix, Biden’s narrow lead in Wisconsin became a 2-point loss, a tie in Pennsylvania flipped to Trump by 6 points and Michigan remained dead even. Such an outcome would return Trump to the White House.

This poll is not an outlier. A Quinnipiac Poll conducted the same week put Biden ahead by 3 points nationally, but when respondents could choose among Kennedy, Stein or West, Trump pulled ahead. In nearly every public presidential survey this year, two truths are evident: Minor party candidates don’t stand a ghost of a chance of winning, and the addition of minor party candidates moves the outcome toward Trump.

That is why we are ringing the third-party alarm. In a race certain to be close, third-party candidates not only pose a problem for Biden, they also may be his biggest hurdle to reelection.

Who is RFK Jr.'s running mate? RFK Jr.'s VP pick shows his presidential bid for what it is: A vanity project all about him

The last two elections showed that Trump has a vote ceiling below 50% of the electorate. He did not reach 47% of the popular vote in either of his presidential races and secured a smaller share of the national electorate in both runs than Mitt Romney did in his loss to Barack Obama in 2012.

In Trump's Electoral College win over Hillary Clinton, he carried the same share of the popular vote as Michael Dukakis and John McCain got in their presidential losses.

Third-party candidates helped Trump in 2016

Trump was aided in 2016 by the presence of minor party candidates, who won a small but decisive 5.6% of the electorate. In 2020, Trump was ruined by their absence.

In a pure head-to-head matchup, Biden would be a prohibitive favorite because enough voters under no earthly circumstances will ever pull the lever for Trump.

But that is not the contest we will see this year. At a minimum, three minor party candidates are actively seeking to appear on the ballot. Every one of these candidates will hurt Biden.

Cornel West and Jill Stein will each run from the extreme left and likely garner a paltry number of votes. Not all of their voters would support Biden, but none of them would support Trump. In 2020, 44,000 votes in three swing states gave Biden the presidency. Stein and West, even with relatively few votes, could put Trump in the White House.

A No Labels “unity ticket” would have been far more damaging. Biden is president because he throttled Trump by 30 points among self-identified moderates, according to 2020 exit polls. A No Labels ticket featuring a Republican and a Democrat could have taken three Biden votes for every two Trump votes, a potentially game-changing boost.

Thankfully, both Chris Christie and Joe Manchin rejected No Labels’ enticements to run, citing their fear of aiding Trump. And the organization announced Thursday that it was dropping its effort to field a presidential candidate.

No Labels drops out of 2024 race. The truth is they were never in it.

Kennedy should have the money needed to stay in the race

Many see Kennedy's candidacy as a wild card. But he’s not − he’s Trump’s ace in the hole. With a Silicon Valley entrepreneur and former wife of a Google billionaire as his newly minted running mate, Kennedy should have the money to stay in the race. He also has the mythical name to appeal to a segment of Democratic voters.

Anyone who believes that Kennedy’s tin foil hat views on issues ranging from vaccines to mass shootings make him more attractive to Trump voters than Biden voters is making a mistake.

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So, what does sounding the third-party alarm mean? Treating minor candidates with the same scrutiny as we use with major party candidates who have a shot at winning. That includes undertaking the extensive opposition research any normal candidate would face; examining their conduct, statements and policy positions; and warning the public that a vote for any of them is a lifeline to Trump.

The lessons from 2016 and 2000 are clear: Minor party does not mean minor impact. No-hope candidates can change the outcome of an election, even by garnering a relative handful of votes.

It’s incumbent on partisans to raise the alarm on today’s crop of spoilers. It’s also vital that the news media make clear to voters that none of these third-party candidates can win – and that supporting them would mean throwing away your vote and helping bring about the catastrophe of Trump’s return to power.

Jonathan Cowan is president and Jim Kessler is executive vice president for policy at Third Way, a center-left think tank.

You can read diverse opinions from our Board of Contributors and other writers on the Opinion front page, on Twitter @usatodayopinion and in our daily Opinion newsletter.

This article originally appeared on USA TODAY: RFK Jr. is Trump's ace in the hole, and ticket back to the White House