Sen. John Fetterman, D-Pa., is reportedly warning against Vice President Kamala Harris picking Pennsylvania Gov. Josh Shapiro as her running mate.

His aides informed Harris' campaign about his concerns — including that Shapiro is too focused on "his own personal ambitions," according to Politico, which cited three people familiar with the matter.

Fetterman and Shapiro came into conflict during their service together on Pennsylvania's Board of Pardons regarding second chances and the commutation process for convicts, the report stated.

Shapiro is one of the leading candidates for Harris' running mate after President Biden announced in July that he would withdraw from the race. 

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John Fetterman and Josh Shapiro split image

One of the Democratic Party's most recognizable figures, Sen. Fetterman, is warning against Vice President Harris picking Pennsylvania Gov. Shapiro as her running mate, according to a new report. (Getty Images)

Shapiro's staff, however, said the governor's history on criminal justice reform is clear. 

"The governor evaluates every pardons case individually and on its merits and during his time as attorney general, he approved more pardons and commutations than all of his predecessors over the last 25 years, combined," Shapiro spokesperson Will Simons told Politico. "He is proud of his record of delivering meaningful criminal justice reform while making Pennsylvania communities safer."

Fetterman still has "full-on, enthusiastic support for Kamala Harris," one of the sources said his team expressed. That source described Fetterman's advisers as giving his "unvarnished assessment" of Shapiro "that is not about policy differences or other public baggage."

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Josh Shapiro, Kamala Harris

Shapiro has also been forced to defend his stance on Israel in recent days as the competition to become Harris' running mate heats up. (Ryan Collerd/AFP via Getty Images)

Shapiro has also been pressured to defend his stance on Israel in recent days as the competition to become Harris' running mate heats up. 

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The Philadelphia Inquirer uncovered the essay that Shapiro wrote for the Campus Times, the student newspaper of the University of Rochester, from which Shapiro graduated in 1995. 

In the 1993 article, Shapiro wrote that "Palestinians will not coexist peacefully," because "they do not have the capabilities to establish their own homeland and make it successful even with the aid of Israel and the United States."

Shapiro, Fetterman and the Harris campaign did not immediately respond to Fox News Digital's request for comment.

Fox News Digital's Peter Aitken contributed to this report.