Sen. John McCain, facing a potential primary challenge in his bid for a new term, is launching two new radio ads today that call him "Arizona's last line of defense."

In one ad, McCain himself accuses President Obama of waging an "extreme left wing crusade to bankrupt America." "I stand in his way every day," the former Republican presidential hopeful then says.

In an email announcing the ads to his supporters, McCain writes that he is "anticipating a competitive reelection race," and the ads are meant to highlight "my efforts to represent our shared values." He also asks for "a generous contribution of $25, $50, $100, $250 or more to help our campaign expand our ad buy."

Chris Simcox, founder of the Minuteman movement, has already announced a challenge to McCain. Former Rep. J.D. Hayworth is also said to be considering a run. The state's primary is August 31.