DES MOINES, Iowa (AP) — Rep. Tom Tancredo dropped out of the race for the Republican presidential nomination Thursday and endorsed rival Mitt Romney.

Tancredo, a five-term congressman from Colorado, had based his campaign on fighting illegal immigration and has run television ads that link lax border control to terrorist attacks, rape and other crimes. He has consistently polled at the bottom of the crowded Republican field.

He announced his withdrawal two weeks before Iowa begins the presidential nominating process with the precinct caucuses.

Tancredo said he decided to drop out in part because of concern that his presence could split the vote for other candidates who have taken a hard line on immigration, helping those who would take a less restrictive approach.

"I fear remaining in this race, one which I cannot win, would contribute to the nomination of one of these candidates," he said.