Gavin Newsom's grave warning to Biden: California governor urges Joe, 81, to take RFK Jr. 'seriously' and reveals why he is worried about the third party candidate

California Gov. Gavin Newsom had a grave warning for President Joe Biden, telling him to take Robert F. Kennedy's presidential bid 'seriously.'

Kennedy just got on the ballot in the battleground state of Michigan and is close to qualifying in other swing states. As an independent candidate, Kennedy must meet the requirements of each individual state in order to be listed on its ballot.

'Bobby Kennedy, Jr. - if he gets on more than just that one critical swing state of Michigan and gets on the ballot in some of these others. I think you have to take that very, very seriously,' Newsom told Jen Psaki on MSNBC on Sunday.

California Gov. Gavin Newsom (above) had a grave warning for President Joe Biden about Robert F. Kennedy Jr.

Polls show Kennedy could take votes away from Biden in November's general election.

'We revere Kennedy's - at least I do,' said Newsom, the Democratic governor of California. He has endorsed Biden for president but said the late Robert Kennedy - RFK Jr.'s father - is the reason he went into politics. 

An NBC News poll from over the weekend showed Kennedy receives enough support among the electorate to play a spoiler in the 2024 election. He polls significantly higher than two other independent candidates: 13% for him versus 3% for Jill Stein and 2% for Cornel West.

But the poll of registered voters suggests that Kennedy's new-found popularity hurts Trump more than Biden. 

In a head-to-head matchup, Trump is two points ahead of Biden, 46 percent to 44 percent. But when Kennedy Jr. on the ballot, Biden shifts ahead of Trump by two points, Biden with 39 percent and Trump with 37 percent. 

Trump had offered Kennedy a spot on his ticket as his running mate, Kennedy claims, adding that he turned the former president down.

Robert F. Kennedy has qualified for the presidential ballot in battlegrounds state of Michigan

President Joe Biden speaks at a campaign rally as the Kennedy family announces their endorsement 

But a CNN poll of Michigan and Pennsylvania last month found Kennedy would play a spoiler in those battleground states - taking enough votes from Biden to propel Trump to victory.

In a head-to-head matchup, voters in Michigan favored Trump with 50% to Biden's 42%. And, in Pennsylvania, it was a tie with 46% for each candidate. 

But when Kennedy and fellow independent candidate Cornel West were added to the mix, the numbers shifted.

In Michigan, voters went 40% Trump, 34% Biden, 18% Kennedy and 4% West, while in Pennsylvania, 40% choose Trump, 38% Biden, 16% Kennedy and 4% West.

Even if Kennedy doesn't make the ballot in all 50 states, many Democrats worry RFK Jr.'s will qualify for enough of the battleground states to hurt Biden on the ballot and hand Trump the White House. 

Biden has not publicly acknowledged Kennedy's entry into the race, although he rallied on Thursday with members of the Kennedy family who enthusiastically endorsed Biden and warned about Trump. 

The president has long admired the Kennedys and tied himself to their legacy at last week's event.

'It's an incredible honor to receive the endorsement of your family. And it means so much to me,' Biden said.

United States Special Envoy for Northern Ireland Joe Kennedy III (center) campaigns fo President Joe Biden after he and 14 other Kennedy family members endorsed Biden for reelection in Philadelphia

At least 15 members of the most-storied political family in America joined Biden at the event in Philadelphia to show their support for their fellow Irish American, including Kennedy's siblings and his nieces and nephews.

Biden has long emphasized his admiration for the Kennedy. He's cited Robert F. Kennedy Sr. as a personal inspiration in his political career. A bust of the former attorney general and senator sits in the Oval Office. 

Additionally, John F. Kennedy was the nation's first Irish Catholic president; Biden is the second. 

The Kennedy clan have signaled their support for Biden for months but Thursday marks their formal endorsement of the president.

Last month, nearly the whole clan joined Biden for a St. Patrick's Day celebration at the White House, splashing a picture with the president across social media.

Kennedys even serve in Biden's administration: Joe Kennedy III is Biden's envoy to Northern Ireland, Caroline Kennedy is ambassador to Australia, and Victoria Reggie Kennedy, the widow of the late Sen. Ted Kennedy, is ambassador to Austria.

Many members of the Kennedy clan joined President Joe Biden at the White House for St. Patrick's Day - and splashed the above photo across social media

Robert F. Kennedy Jr. responded to the news of his family's endorsement with a post on social media.

'I hear some of my family will be endorsing President Biden today. I am pleased they are politically active — it’s a family tradition. We are divided in our opinions but united in our love for each other,' he wrote. 

'I hold this as a possibility for America too. Can we disagree without hating our opponents? Can we restore civility and respect to public discourse? I think we can.'