(NYT) Months ago, the president of CNN/U.S., Jonathan Klein, spelled out two options for Lou Dobbs, the network’s most outspoken anchor. Dobbs could vent his opinions on radio and anchor an objective newscast on television, or he could leave CNN altogether.

On Wednesday, Dobbs made his decision: He chose opinion.

Dobbs told viewers that he was resigning from his CNN job immediately. Sitting before an image of an American flag on his studio set, he said “some leaders in media, politics and business have been urging me to go beyond the role here at CNN and to engage in constructive problem-solving as well as to contribute positively to the great understanding of the issues of our day.”

He remained vague about how he would contribute “to the national conversation,” saying that he was considering “a number of options and directions.”

The abrupt announcement caught even some of his closest staffers by surprise. They were told about the decision only hours before Dobbs’ 7 p.m. broadcast.

CNN said it would name a replacement for Dobbs on Thursday morning. Klein said in a statement that “Lou has now decided to carry the banner of advocacy journalism elsewhere.”

“All of us will miss his appetite for big ideas, the megawatt smile and larger than life presence he brought to our newsroom,” he said. Klein and Dobbs declined interview requests.

Well known for his opposition to illegal immigration and his fierce advocacy of free-market capitalism, Dobbs was an outlier at CNN, which has sought to be seen as the neutral turf of cable news.

“If CNN wants to be seen as the thoughtful, unbiased, middle of the road alternative to Fox News on the right and MSNBC on the left, this decision goes along with that,” said Geneva Overholser, the director of the School of Journalism at the University of Southern California. She said the anchor’s decision also makes sense “if he really wants to be Lou Dobbs: Man of Opinion.”

Dobbs’ show drew an average of 631,000 viewers in October, putting him in third place behind Fox News and MSNBC. Like those of other CNN programs, Dobbs’ ratings have declined in recent months.

His contract was not set to expire until the end of 2011, people with knowledge of his deal said. They spoke on condition of anonymity because contract information is typically treated confidentially. Dobbs told viewers that Klein had agreed to release him from his contract early.

People inside CNN have speculated for months that Dobbs could pick up and move to another network, a possibility that gained momentum in September when he met with Roger Ailes, the chairman of the Fox News Channel. At the time Dobbs was characterized by people close to him as a potential hire for the Fox Business Network. But a Fox spokesperson said Wednesday, “We have not had any discussions with Lou Dobbs for Fox News or Fox Business.”

Dobbs quit CNN once before, in 1999, after disputes with the network’s president at the time, Rick Kaplan. He became the chief executive of Space.com, an astronomy news source, but returned to the network two years later.

Over the years Dobbs evolved from being a sober business anchor to a commentator who labeled himself “Mr. Independent.” The issues at the top of his agenda – ones that he mentioned again on Wednesday in explaining his departure – include immigration policy, the growth of the country’s middle class, climate change, and the military involvement in Afghanistan and Iraq.

He had saved his most ideological remarks for his afternoon radio show, which made its debut in March 2008. It is on the radio show that he talked repeatedly about the conspiracy theory claims that President Barack Obama is not a U.S. citizen. When he mentioned the citizenship issue on CNN over the summer, he raised the ire of his bosses and provoked criticism from progressive groups.