The sleepy countdown to the 2024 presidential primaries got a jolt on Monday when Robert F. Kennedy Jr. dropped his primary challenge to President Joe Biden and announced plans to run as an independent -- a move that triggered a barrage of attacks from former President Donald Trump and Republicans worried over polling that shows the longtime Democrat could gnaw into Trump's support.

Kennedy's decision opened up a new and volatile chapter in a campaign that has been bogged down by the seeming inevitability of Biden and Trump, who appear poised to run away with their parties' nominations. The GOP, though, is not alone in their anxiety. The prospect of at least two more independent candidates on the ballot next year has created an unprecedented new level of uncertainty for both parties.

Like Kennedy, Cornel West, the progressive scholar and activist who was poised to claim the Green Party's nomination, revealed last week he too would go it alone.

And then there is the persistent specter of a centrist, third-party bid from No Labels, the well-funded group considering a ticket of its own.

Following a pair of presidential races decided by just tens of thousands of votes in a handful of battleground states, the unpredictability of candidates from outside the two-party system has ushered in a heightened sense of uncertainty over the trajectory of the coming general election campaign. While the consensus largely remains that Biden has more to lose from the presence of independents on the ballot, Kennedy's departure from the Democratic primary has some pro-Trump Republicans eyeing him more warily.

Though there is no consensus over how Kennedy's campaign would affect Trump's prospects, Republicans allied with the former president have nonetheless been building opposition research against the longtime Democrat, intending to go on the offensive and paint Kennedy as a "liberal parading in conservative's clothing," one adviser told CNN, pointing to his past record as an environmental activist.

Voters got a taste of what's to come on Monday when the Republican National Committee issued a statement, just prior to Kennedy's announcement, characterizing him as "just another radical, far-left Democrat."

"Voters should not be deceived by anyone who pretends to have conservative values. The fact is that RFK has a disturbing background steeped in radical, liberal positions," Trump campaign spokesperson Steven Cheung told CNN after RFK's announcement, pointing to RFK's stance on a variety of social issues. "... A RFK candidacy is nothing more than a vanity project for a liberal Kennedy looking to cash in on his family's name."

Others believe Kennedy's pull with voters who "hate the system," as one person close to the former president characterized it, could ultimately pose a significant problem for Trump, who typically dominates that bloc.

Another conceded that Kennedy could be a real threat to Trump, particularly when it comes to vaccines.

A bloc of conservatives, including many of his core supporters, have lambasted the Covid-19 vaccines as experimental and dangerous. The vaccines, which were created and expedited during Trump's administration, were arguably one of his top achievements while in office and have caused the former president to walk a tightrope during his latest bid for the White House. In a recent interview with Megyn Kelly, Trump defended the creation of the vaccine, saying he was "not proud of it" and insisting he never "gave mandates."

"If you're a single-issue voter on Ukraine, you now have two options instead of one. His views on vaccines are also a big pull to those kind of voters," another adviser said. "Those are people that like are less likely to tell pollsters that they're for Trump. They're less likely to answer the phone for pollsters."

Trump had initially praised Kennedy after he launched his Democratic primary bid.

"He's a very smart person," the former president said on Fox News in June. "I know a lot of the members of that family, and he's a very smart guy. And he's hit a little bit of a nerve. And a lot of Democrats I know want to vote for him."

As it turns out, according to the polls, not very many did -- at least not in a race with Biden for the Democratic nomination. Kennedy never cracked 20% in a credible survey of Democratic voters, likely hastening his decision to break from the party that still celebrates his late father and uncles.

Kennedy's candidacy, which has mostly made headlines for his vaccine skepticism and Covid-19 conspiracy theories, is widely viewed by top Democratic strategists as, at worst, more of an annoyance than a threat.

Even in New Hampshire, where Kennedy had hoped to capitalize on Biden's likely absence from the primary ballot after a dispute over his efforts to revamp the 2024 nominating calendar, Democratic activists had already begun planning a write-in campaign on the president's behalf.

"Granite staffers take their first-in-the-nation primary really seriously, and we're not taking any chances. We're going to make sure [Biden's] the nominee," Matt Wilhelm, the Democratic leader in the New Hampshire House told CNN last week. "I'm frustrated but I'm not so bitter that I'm willing to throw the Biden out with the bath water."

Unlike the RNC, The Democratic National Committee did not issue a statement following Kennedy's announcement Monday.

West's candidacy had been causing more tremors among some Biden operatives for what they believed was the celebrity scholar's potential as a Green candidate to shave off enough Black and liberal votes in swing states to make him a spoiler.

Biden's top advisers, though, have not quite bought into that fear themselves, even as some donors and Biden-aligned independent groups are preparing to push back against West.

Now as both Kennedy and West relaunch as independents, Biden campaign aides say they're confident that many voters are still haunted by memories of how third-party candidates won more votes than Trump's and George W. Bush's margins of victory in key states in 2016 and 2000, respectively.

The Biden campaign declined to comment. A person close to the president's reelection effort reiterated a belief that this will be "a very close election, decided by a small number of voters in a small number of states."

But the person added: "The president's team has multiple paths to victory and is focused on turning out their voters, persuading their persuadables, and building the coalition that will send Biden back to the White House."

West's decision to break from the Green Party, which would have offered him more in the way of ballot access and campaign infrastructure, is likely to calm the nerves of Democrats anxious over his appeal. Those same supporters and party officials got a boost of sorts from an unlikely source last week, when Ralph Nader, the celebrated activist and former Green presidential nominee, spoke publicly about his support for Biden.

"Fascism is what the GOP is the architecture of, and autocracy is what the Democrats are practitioners of," Nader told The Washington Post. "But autocracy leaves an opening. They don't suppress votes. They don't suppress free speech."

Given the choice, Nader said, "I'll take autocracy any time."

Hardly a ringing endorsement of the president and Democratic Party, but his words did not pass unnoticed by West's campaign.

West campaign manager Peter Daou told CNN that while Nader did not reach out directly to encourage West to leave the Greens, West holds Nader in the highest regard. Daou also denied that Vermont Sen. Bernie Sanders, a longtime political and ideological ally of West's, had made any attempt to influence the candidate's decision.

Sanders, who endorsed Biden in April, has not responded to a request for comment on West's move, though multiple former top Sanders aides told CNN this summer they opposed West's Green Party run and were not clear on his aims.

Briefly addressing his friend's run earlier this year, Sanders was hardly encouraging, saying, "People will do what they want to do."

The greater source of concern for Democrats remains the potential of a No Labels ticket, which they fear -- with polling data to back them up -- could curry support with centrist and center-right voters who would never vote for Trump but are not enthusiastic about Biden.

Third Way, which has been leading the charge against No Labels for Democrats, has had only informal and indirect contact with the Biden campaign and the president's top advisers. The DNC's involvement so far has been participating in a couple of video calls Third Way has hosted with groups of state Democratic Party officials, according to Matt Bennett of Third Way.

The leader of No Labels insist they have no desire to play spoiler and have not yet determined if they will field a candidate even as they seek ballot access across the country.

"I've never been in any race I've ever spoiled. I've been in races to win," West Virginia Sen. Joe Manchin, a conservative Democrat rumored to be considering a presidential run, said at a No Labels forum in July. "And if I get in a race, I'm going to win."

No Labels CEO Nancy Jacobson, too, has repeatedly said in interviews that she has no desire to tip the election to either major party and that the group would ultimately make a decision over its next steps based on polling, data and the results of the GOP primary.