Will Robert F. Kennedy Jr. spoil the election for Biden — or Trump?

No sooner had Robert F. Kennedy Jr. announced he was running for president as an independent than Republicans and Democrats alike started fretting about the possibility that he would siphon votes away from them in 2024.

"Voters should not be deceived by anyone who pretends to have conservative values," a spokesperson for former President Donald Trump said of Kennedy. And a co-founder of the center-left group Third Way similarly expressed concern about the third-party bid, telling the Wall Street Journal, "Our take on it is that anything that divides the anti-Trump coalition is bad."

Both sides have valid reasons for concern; there's some evidence to suggest that Kennedy would take more votes away from the Democratic presidential nominee, and some evidence to suggest that he might take more votes away from the Republican nominee. But at the end of the day, his impact on the presidential race is probably being overstated. Third-party candidates rarely win a significant share of the vote, and the election would have to be extremely close for Kennedy's presence to change who actually wins.

Why Kennedy could hurt Democrats

Only a few polls so far have tested a hypothetical three-way race between Kennedy, Trump and President Biden, but they've mostly found the same thing: Kennedy's presence slightly increases Trump's margin over Biden. On average, Trump leads Biden by only 0.5 percentage points in national polls when Kennedy isn't included, but that lead grows to an average of 1.8 points in the three-way matchups.

There are a couple reasons why this could be. First, until last week, Kennedy himself was a Democrat, running against Biden in the Democratic primary. He is also a member of the famous Kennedy clan, which has produced titans of the Democratic Party such as former President John F. Kennedy (his uncle) and former Massachusetts Sen. Ted Kennedy (another uncle). Voters who recognize his name and are nostalgic for leaders like them might be inclined to vote for Kennedy — and those voters are probably mostly Democrats.

Democrats are also less enthusiastic about their likely presidential nominee than Republicans are about theirs, meaning they may be more open to voting for an alternative. According to a recent poll from Marist/NPR/PBS NewsHour, only 30 percent of Democrats and Democratic-leaning independents said they would be "very satisfied" if Biden is the Democratic nominee in 2024, while 43 percent of Republicans and Republican-leaning independents said they would be very satisfied if Trump is the GOP's pick. Meanwhile, Trump is currently sitting at 58 percent in 538's national polling average of the Republican primary. That's basically the same as Biden's support in our national polling average of the Democratic primary, despite the fact that Trump is facing much stronger competition.

Why Kennedy could hurt Republicans

On the other hand, we shouldn't take those three-way polls as gospel. First of all, the shifts toward Trump are tiny — well within the polls' margins of error, which means they could just be noise in the data. (However, the fact that four separate surveys all found roughly the same thing does make us more confident that the pattern is real.) Additionally, even if those polls suggest that Kennedy's support is mostly coming from Democrats today, that doesn't mean that will be the case come November 2024. Polls of the general election taken so early in the cycle have historically not proven very accurate.

In fact, there's one good reason to think that Kennedy could actually hurt the GOP nominee more than the Democratic one: He's a lot more popular with Republicans than with Democrats. In the past month, five pollsters have conducted polls on Kennedy's favorable and unfavorable ratings broken down by party. On average, his net favorability among Republicans is +27 points (55 percent favorable, 27 percent unfavorable) — but his average net favorability among Democrats is -10 points (35 percent favorable, 45 percent unfavorable).

That's because Kennedy holds a number of beliefs that put him closer to the Republican base than the Democratic one. Most famously, he is a vocal skeptic of vaccines, but he has also said that gun control does not meaningfully reduce gun violence and opposes aiding Ukraine in its war against Russia.

Over the course of the next year, as Kennedy campaigns, voters will become better acquainted with his positions, which may discourage some Democrats from supporting him (and/or encourage some Republicans to do so). We've already seen something similar happen over the course of Kennedy's Democratic primary campaign: His net favorability rating among Democrats dropped over the course of the summer the more they got to know him.

Why Kennedy's campaign may not matter much after all

At first glance, Kennedy looks like he could be a big factor in the 2024 presidential race. In each of the five national polls that tested a three-way matchup between him, Biden and Trump, he got between 12 and 19 percent of the vote. That would make him the most successful third-party presidential candidate since Ross Perot.

But Kennedy will most likely not reach those lofty heights. Support for third-party candidates tends to decline over the course of the campaign, as voters get skittish about casting a ballot that "won't matter" and retreat to their preferred (or least hated) major-party candidate. According to Gallup, independent John Anderson was polling at 21 percent at the end of March 1980, but he was down to 8 percent by late October, and he ended up winning less than 7 percent of the popular vote. And according to 538's polling average of the 2016 election, Libertarian Gary Johnson was polling at almost 10 percent in mid-July 2016, but by late October he had fallen to 6 percent, and he ended up winning just 3 percent.

Plus, the conflicting data over whether Kennedy might take more votes away from Democrats or Republicans could portend that he will take roughly equally from both parties — which of course wouldn't affect the ultimate winner. And in past elections with prominent third-party candidates, a majority of their supporters told exit pollsters that, if it had been a head-to-head race, they simply would have stayed home.

Add it all up, and the net number of votes that Kennedy would cost Biden or Trump is probably extremely small. For example, let's say that Kennedy wins 5 percent of the popular vote next year — which is pretty generous, considering that no third-party candidate has received that much support in over 25 years. Let's also say that, in a head-to-head race, 30 percent of Kennedy's supporters would have voted for Biden, 20 percent would have voted for Trump and 50 percent wouldn't have voted at all. In this scenario, Kennedy would have cost Biden 1.5 points (30 percent of 5 points) of the overall vote and would have cost Trump 1 point (20 percent of 5 points) of the overall vote. In other words, Kennedy's net effect on the election in this scenario would have been to decrease Biden's margin by 0.5 points.

The race between Biden and Trump would have to have been extremely close for this to change the winner. That's certainly possible in this era of closely contested elections: In 2020, the tipping-point state in the Electoral College (Wisconsin) was decided by just 0.6 points. But most elections just aren't that close.

When analysts and political scientists have investigated the question of whether third-party candidates have affected who won elections, they have almost always found that the answer is no: not in the 2016 presidential race, not in the 1992 presidential race, not in the 2013 Virginia gubernatorial race and not in the 2010 or 2014 Maine gubernatorial races.

The most prominent exception is the 2000 presidential race, which Green Party candidate Ralph Nader probably did spoil for former Vice President Al Gore — but only because that election was so extraordinarily close. Florida, the decisive state that year, was decided by just 0.009 percentage points. So for Kennedy to spoil the 2024 election for either Biden or Trump, he's going to need a perfect storm of circumstances.