President Trump is attempting to use attorney Robert Kennedy Jr.’s vice presidential pick, Nicole Shanahan, to paint the independent presidential candidate as “Far Left” and to shore up support among his supporters who might be Kennedy curious.

Mr. Kennedy on Tuesday announced that Ms. Shanahan, a wealthy attorney from California, would join his independent presidential ticket, saying at a rally: “I found exactly the right person.”

Now, Mr. Trump, in a post on Truth Social, is doubling down on his attacks against Mr. Kennedy, calling him a “Radical Left Democrat” and attacking Ms. Shanahan.

“His running mate, Nicole Shanahan, is even more ‘Liberal’ than him, if that’s possible,” Mr. Trump said. “It’s great for MAGA, but the Communists will make it very hard for him to get on the Ballot.”

Mr. Trump went on to say that observers should expect “him, and her, to be indicted any day now, probably for Environmental Fraud.”

“He is Crooked Joe Biden’s Political Opponent, not mine,” Mr. Trump said. “I love that he is running.”

Mr. Trump’s attacks have only escalated since Mr. Kennedy announced that he would leave the Democratic Party and run as an independent last year.

Since then, Mr. Trump has been working to sway his supporters against Mr. Kennedy even though the independent candidate’s signature issue, attacking the safety of vaccines, resonates far more with Republican voters than with Democrats.

A Morning Consult and Politico survey from last fall, taken at about the same time Mr. Kennedy announced his independent run, found that 51 percent of Republicans cared more about the potential risks of vaccines than their benefits. Among Democrats, the number was 24 percent.

The same survey found that Republicans who supported Mr. Trump in the primary were twice as likely to think that vaccines were unsafe for children compared to Republicans who did not support Mr. Trump in the primary.

The dynamic has left Mr. Trump unable to take credit for any of the successes of the Covid vaccines and has made the whole issue a vulnerable subject for the former president among some of his most vocal supporters.

This dynamic played out in a post from Mr. Trump in early March, when the former president tried to take credit for the Covid vaccines, saying, “YOU’RE WELCOME, JOE, NINE MONTH APPROVAL TIME VS. 12 YEARS THAT IT WOULD HAVE TAKEN YOU.”

The post met with a backlash from his followers, suggesting that, for at least some Republicans, Mr. Kennedy could use the issue as a point of leverage.

At the same time, general election polling among Mr. Trump, President Biden, and Mr. Kennedy has found that having Mr. Kennedy in the race appears to hurt Mr. Biden more than Mr. Trump.

In a recent survey from the Hill and Decision Desk HQ, Mr. Trump enjoyed 40.5 percent support in a three-way matchup, with Mr. Biden carrying 38.6 percent and Mr. Kennedy at 9.8 percent.

While there is still time for both campaigns to mobilize their messaging against Mr. Kennedy, not to mention that it’s not yet clear where Mr. Kennedy will be on the ballot, members of the Kennedy clan are already expressing concerns that he may spoil the election for Mr. Biden.

Mr. Kennedy’s sister, Rory Kennedy, told CNN that she thinks Mr. Kennedy is “siphoning” votes from the president and that he could “shift the election and lead to Trump’s election.”

“I feel strongly that this is the most important election of our lifetime. And there’s so much at stake, and I do think it’s going to come down to a handful of votes and a handful of states,” Ms. Kennedy said.