As U.S. Sen. John McCain readies his official announcement of a bid for a sixth term, national "tea party" organizations are stepping up their efforts to find a viable conservative opponent to challenge him in Arizona's 2016 Republican primary.

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Over the years, McCain has taken positions that many tea-party conservatives oppose: most notably his support of comprehensive immigration reform, which critics say would provide "amnesty" to illegal immigrants, and of the Troubled Asset Relief Program, which they viewed as a big-government bailout.

Besides being a top target of big spending by out-of-state groups, McCain also must manage a long rocky relationship with his local conservative base. Since Arizona Republican Party activists formally censured him as too liberal in January 2014, McCain and his establishment allies have made progress in moderating the state GOP's internal politics by encouraging like-minded Republicans to run for party posts.

Appearing at this year's state GOP meeting on Jan. 24, McCain was greeted by a noisy mix of boos and cheers. About a dozen people stood with their backs to McCain while he spoke.

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