Democratic Rep. Ro Khanna of California called on Nicole Shanahan, the running mate of presidential candidate Robert F. Kennedy, to step down, warning that supporting Kennedy could pave the way for former President Donald Trump to win the election. 

He made his pitch to her in a letter he shared with CBS News, though he hadn't yet sent it to Shanahan. 

"Even Trump himself, and other members of his team, have admitted that a RFK Jr. ticket will help his reelection," Khanna wrote in his letter.

"While you may have fair disagreements on the Democratic Party's platform, it is clear that a second term for Trump would be disastrous for climate and undo the work of President Biden's Inflation Reduction Act, the most significant action Congress has taken on clean energy and climate change in our nation's history," Khanna went on. 

When reached by CBS News, Shanahan opted to post her response to the letter on social media, making it clear she had little patience for Khanna's latest thoughts on the Kennedy campaign. 

"In my conversation with Ro he congratulated me on the position and encouraged me to run, stating that every American has the right to run in this country," Shanahan wrote on X. "He stated that we live in a democracy, and it was wrong for anyone to threaten me against running."

"Clearly, Ro has changed his stance based on pressure from the party," she continued. "I hope he understands how anti-democratic it is to ask someone to step down from a race that empowers the American public to make their own decisions." 

She expressed disappointment that he had not called her privately, and said his actions were "performative."

"He has my direct line," she wrote. 

Shanahan said her post was her "full response to CBS" and she was writing it to show "I am beholden to you the people and not the corporate press."

Khanna, a surrogate for President Biden's reelection campaign, wrote in his letter to Shanahan that recent polling suggests that in swing states, Kennedy could tilt the November election in Trump's favor, and he advised her to consider the potential impact of another Trump presidency on the environment, which was an issue Shanahan highlighted in her decision to join the Kennedy campaign. 

Shanahan called herself a "disillusioned Democrat" late last month during the revelation that she would be Kennedy's running mate, telling voters at the Oakland, California, event that she had contacted several political figures to discuss environmental policy, but "none of them take any action." This, she said, prompted her to join Kennedy's ticket.

February financial campaign filings show that Shanahan, a wealthy California-based attorney, had already donated $4 million to support the Kennedy campaign, which helped fund a Super Bowl ad for the independent longshot. 

Shanahan has also donated significant funds to Khanna, more than $17,000, his office said, with the most recent donation coming last year. According to Khanna, their shared belief in protecting the environment led to their introduction, and they have known each other for years.

Democrats harbor some concerns about Mr. Biden's ability to defeat Trump in battleground states where the margin of victory in 2020 was very small. Arizona, Nevada, Georgia and Michigan were all states that Mr. Biden won by under 50,000 votes. 

Some Democratic groups have been trying to stop third-party candidates from mounting bids against the president. Democratic groups MoveOn and Third Way announced last week that they planned to shift their focus to weakening Kennedy after the group No Labels announced it would no longer pursue a competing Unity ticket. 

"Nicole, of course, I want to be clear, I respect any person's right to run," Khanna told CBS News by phone Tuesday. "I respect her. I completely respect third parties and multiple parties, but I was just making the case from a perspective of persuasion." 

"I certainly don't want to say anything that is negative about her personally, but I would hope she would see the value of joining the broader Democratic coalition," he added. 

After trying to dissuade Shanahan to join Kennedy's campaign privately, Khanna's office said it decided to publicize the letter to push her to reconsider.

"Rep. Khanna decided to make this letter public and alert press to help bring attention to the dangers that RFK's campaign poses," said Marie Baldassarre, a spokeswoman for Khanna. "He also reached out to Nicole privately previously to urge her to reconsider and join the Biden coalition." 

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• RFK Jr.