Republicans' impeachment efforts are paying off — for President Joe Biden.

A fundraising email from Kamala Harris attacking House Republicans for voting this week to authorize a "ridiculous" impeachment inquiry has already become the vice president's top-performing pitch of the entire reelection, a Biden campaign source told CNBC.

It was also the campaign's best pitch of the month, added the source, who requested anonymity to discuss the fundraising development.

It's "a testament to the energy against this whole thing," said the source, who declined to say exactly how much had been raised.

"This inquiry is nothing but political theater with bad actors," the email read. Harris accused the Republicans of trying to "destroy President Biden and our record."

The impeachment-focused campaign pitch takes a page out of Donald Trump's playbook, by capitalizing on a message of political grievance and presenting the president as a victim of unfair attacks.

Trump, the leading candidate for the 2024 Republican nomination, regularly decries his many active criminal and civil lawsuits as witch hunts and political "lawfare." His campaign references those legal battles in near-daily fundraising pitches that accuse Biden of trying to suppress his top political rival.

Despite Biden's low approval ratings, his campaign has already raised massive sums toward his bid for a second term. His fundraising operation raked in over $71 million in the third quarter, far exceeding the totals posted by Trump and other Republican presidential contenders.