Republican presidential candidate Tom Tancredo plans to drop out of the race during an announcement Thursday afternoon in Des Moines, Iowa, GOP sources close to the campaign have told FOX News.

Tancredo, who has aggressively pushed his anti-illegal immigration message throughout the GOP primary race, has not been able to break through in the polls against his better known competitors. He and California Rep. Duncan Hunter typically poll in the single digits behind Texas Rep. Ron Paul in national surveys.

Tancredo would not personally confirm whether he will pull out of the race.

"I will neither confirm nor deny that report," Tancredo told FOX News. But he  added, "I wouldn't have a press conference if I didn't have anything to say."

The five-term Colorado representative already announced in late October that he would not seek reelection to the House of Representatives in 2008. He said at the time that his core issue of illegal immigration has been elevated to national prominence and that he doesn't need to remain in Congress to promote it.

Tancredo aired a startling ad a month ago tying illegal immigration to terrorism. The ad showed a man in a hooded sweatshirt and backpack walking into a crowded shopping mall, with the sound of an explosion ringing out after the screen goes dark. The ad then showed bloody pictures of victims of terrorist acts in Europe, while the narrator said: "The price we pay for spineless politicians who refuse to defend our borders against those who come to kill."  

Tancredo focused on the issue at the Republican presidential debate a week ago in Iowa, saying immigration without assimilation is a "catastrophe."

He also refused to participate in a Spanish-language debate on Univision in early December. He told FOX News he did not want to encourage immigrants in America to speak Spanish as their primary language.

FOX News' Carl Cameron and Chad Pergram contributed to this report.