"I entered this race as a voice to win back the places we lost, bridge divides and rid our system of the corrupting influence of dark money," Bullock said in a statement. "While the concerns that propelled me to enter in the first place have not changed, I leave this race filled with gratitude and optimism, inspired and energized by the good people I've had the privilege of meeting over the course of the campaign."

"While there were many obstacles we could not have anticipated when entering this race, it has become clear that in this moment, I won't be able to break through to the top tier of this still-crowded field of candidates," Bullock said.

"While he plans to work hard to elect Democrats in the state and across the country in 2020, it will be in his capacity as a governor and a senior voice in the Democratic primary — not as a candidate for U.S. Senate," the spokeswoman, Galia Slayden, said in a statement.

In six months in the race, Bullock focused almost entirely on building a winning campaign in Iowa. He attracted support from Iowa Attorney General Tom Miller (D), a personal friend and the most senior elected official to offer an endorsement of a 2020 contender.

Miller also appeared in one of two TV ads Bullock ran in Iowa last month, at a cost of about $500,000.

But he could not translate that into a broader base of support. Bullock scored more than 1 percent support in only one poll conducted in Iowa this year, an Emerson survey conducted in October.

He qualified for only one of the Democratic debates, where he failed to score a breakout moment. His campaign had raised $4.3 million through the end of the third quarter of the year, putting him well behind the front-running candidates.

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