The Senate Conservatives Fund (SCF) has launched an effort to defeat Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (R-Ariz.), bashing him for failing to uphold the principles of a “true conservative” just hours after he announced his intention to run for reelection.

“There are few Republicans who have betrayed our conservative principles more than John McCain,” SCF President Ken Cuccinelli said in an email to supporters Tuesday of the party's 2008 nominee for president.

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“John McCain lost his way a long time ago and it's time to replace him with a strong conservative leader who will support and defend the Constitution.”

Cuccinelli’s email links to a petition to replace McCain, which, as of 2 p.m. ET, has 457 signatures. He slams McCain for high-profile instances where he crossed the aisle, including helping to forge the bipartisan Senate immigration bill, as well as voting for various fiscal cliff and debt limit deals.

McCain is set to announce his decision to run again during a speech at an Arizona Chamber of Commerce luncheon in Phoenix. He teased the decision in an interview with NBC News posted late Monday night.

"I'm ready. I am more than ready. In some ways, I am eager,” he told NBC’s Kelly O’Donnell. “I think I have the knowledge and background to help the nation through very perilous times, and I believe that I can still contribute in many ways to the welfare and benefit of my state.”

The SCF isn’t the first conservative group to target McCain. The president of the Club for Growth said the group might support a primary challenger to the five-term senator. And Reps. David Schweikert David SchweikertHouse Democratic campaign leader predicts bigger majority Democrat Hiral Tipirneni wins Ariz. primary to challenge Rep. David Schweikert Ethics watchdog finds 'substantial' evidence of improper spending by Rep. Sanford Bishop MORE and Matt Salmon Matthew (Matt) James SalmonCOVID-19's class divide creates new political risks Arizona voters like Kyl but few think he'll stick around Former Sen. Jon Kyl to replace McCain in Senate MORE (R-Ariz.) both told The Hill in February that they’re eyeing a bid to unseat McCain.

McCain was once the standard-bearer of the party during his run for president. He struggled at times with the party’s base during the primaries, mainly over his immigration position.

Since the election, he’s remained a prominent voice within the party, specifically on foreign policy issues. He now serves as the chairman of the Senate Armed Services Committee.