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TALLAHASSEE, Fla. -- U.S. Rep. Connie Mack IV is blasting U.S. Sen. Bill Nelson for going along with President Obama decision opposing the Keystone XL pipeline project that would carry oil through the nation's midsection.

Mack's campaign to win the Republican nomination and unseat Nelson released its first radio ad Tuesday criticizing Nelson and the president as "lockstep liberals" who gave in to "left-wing environmental extremists" for blocking the pipeline project earlier this year. Obama has said he's willing to reconsider the decision if the pipeline could be rerouted further away from a Nebraska aquifer that provides water to several surrounding states.

Mack's ad seeks signatures for a petition he's begun in hopes of getting Washington to build the pipeline. He does not mention his opponents for the GOP nomination.