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WASHINGTON - Being a U.S. senator is not as attractive as it used to be.

Gov. Dave Heineman of Nebraska said today he would not be a candidate for the Senate seat in his state that will become vacant next year because Sen. Mike Johanns is not running for a second term.

"After careful consideration of all of the issues involved in a race for the United States Senate, I have decided to keep my focus on being the best governor that I can for the citizens of Nebraska," Heineman said in a letter to his supporters. "Every day, I enjoy the challenges and opportunities of being the governor of the best state in America."

Heineman, a popular Republican governor who could have almost certainly waltzed to victory in 2014, is the latest in a string of prominent elected officials who are taking a pass on running for the Senate. Several veteran incumbent senators have also announced their retirements, with some saying they are frustrated by the dysfunction in Washington.

Senate Republican leaders in Washington have spent months trying to persuade Heineman to run. While there is little danger that the seat could slip into Democratic hands in the deeply-conservative state, there is no room for error in the GOP's quest to win control of the Senate next year.

Even the prospect of serving as a senator in the Republican majority, should the party pick up six seats and assume control from Democrats, was not attractive enough to Heineman, aides said. He is among a long list of governors who have taken a pass on joining Congress after being a chief executive of a state.

Johanns, a former governor and agriculture secretary in the administration of President George W. Bush, announced in February that he would not run for a second term next year. The decision from Heineman opens the door to a wide-open race in Nebraska, where there is also a competitive governor's race next year.