WASHINGTON - Don Bivens, a top Democratic challenger in Arizona's U.S. Senate race, announced Wednesday that he was ending his campaign because a competitive primary battle was draining resources the party needed to win in November.

Bivens' withdrawal clears a path in the Democratic primary for former Surgeon General Richard Carmona. While Bivens, a former state party chairman entered the race first, Democratic officials in Washington made clear they believed that Carmona stood the best chance of winning a general election.

"While I am confident we would win this primary, the cost and impact on the party I've spent my life fighting for could diminish our chance to achieve the ultimate goal - winning in November," Bivens said in a written statement.

Democratic leaders in Washington praised Bivens, but they did him few favors by advocating so aggressively for Carmona.

"Don Bivens has had a diverse and distinguished career in business, law and public service," said Sen. Patty Murray, chairman of the Democratic Senatorial Campaign Committee. "His leadership in our party has been critical to our success in Arizona. I know that Don will continue to contribute to our party and our country in the months and years to come."

National Republican Senatorial Committee communications director Brian Walsh issued a statement saying it should be no surprise that Bivens exited the race, since President Barack Obama personally recruited Carmona to run.

Walsh said Wednesday's news should remind Arizonans of Carmona's ties to the president, who they criticized for what they called a liberal, big government agenda.

"We look forward to hearing when President Obama and Richard Carmona will be campaigning side-by-side in Arizona in the days ahead."

Obama did speak with Carmona last year and urged him to run, but that came after months of exploration about a possible candidacy, Carmona spokesman Andy Barr said.

"Having the party united going forward, it's a very good path to victory for us, as Democrats," Barr said of Bivens' decision.

Carmona called Bivens "a cornerstone of the Arizona Democratic Party," thanked him for his service and said he will be an important player in the race.

"Don knew how great of an opportunity we have to win this seat and knew that it would take a strong and united effort to do so," Carmona said in a statement. "He and his team ran a strong race that showed a great deal of respect for the nominating process, which in turn has improved our party's chance of success in November."

Democrats have targeted Arizona as an opportunity to win a seat now held by retiring Republican Sen. Jon Kyl.

Rep. Jeff Flake and businessman Wil Cardon are competing in Arizona's Republican primary. Flake is viewed as the heavy favorite.