Republican presidential candidate Tom Tancredo announced Thursday that he is dropping out of the race, and that he will endorse rival Mitt Romney for the GOP nomination.

He made his announcement at a news conference in downtown Des Moines, two weeks before Iowa begins the presidential nominating process with the caucuses Jan. 3.

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

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.

On Thursday, Tancredo said: "While several of the candidates appear to be committed to our cause, there are others whose records as public servants are abysmal on this issue. They have been on the other side of the battle for years and encouraging and even inviting, in a way, illegal immigrants into the country. I fear that remaining in this race, when I know I cannot win, could contribute to the nomination of one of those candidates. We have done too much. We have come too far. The stakes are way too high for me to allow that to happen."

"And so today I'm doing two things that I believe are in the best interests of this cause, and that cause is, of course, a secure America. I am withdrawing from the race and I'm endorsing Governor Romney for president of the United States," Tancredo continued.

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.

Tancredo said he was backing the former Massachusetts governor because he believes Romney will give due attention to the immigration issue.

"I am convinced that he will do the things he said he will do in the areas that I -- specifically in the areas that I pointed out.

 And there are things that we still disagree on.  But you know what?  There are things that I wish I could have gotten all the candidates to agree with me on, and that's not there," he said.

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