DENVER -- Rep. Tom Tancredo, R-Colo., said Wednesday he will no longer be bound by his pledge to limit himself to three terms.

In a letter to supporters, Tancredo (pictured, left) said he made the decision after concluding the United States is facing a major challenge.

The letter was obtained by The Associated Press. Tancredo campaign spokesman William Mutch confirmed the letter.

In a telephone interview, Tancredo said he and other members of Congress who took the term-limit pledge were hoping to spark a revolution and get 50 others to sign, but it did not catch on as expected.

"I don't know of anyone who is still committed to it. At the congressional level, it's no longer a driving force. It's unworkable," he said.

Tancredo said he decided he had additional work to do in Congress after he gained national attention as one of the most vocal critics of the government's immigration policy.

"Events and fate have cast me into a situation of leadership with regards to the immigration issue. It will determine what kind of nation we will be, either united or divided, and it will determine if we will be a nation," he said.

In the Nov. 5 election, Tancredo is seeking his second term representing the 6th Congressional District.

"It is my deeply held, and now tremendously reinforced, belief that our nation is confronted with a physical, spiritual and philosophical threat that will require every ounce of our individual effort in its defense," Tancredo said in his letter.

"If I am elected to a third term, any decision to run for a fourth will not be made solely on the issue of term limits, but will be weighed against a number of things including the continued existence of a 'fire in my belly' for the challenge.

"If I conclude that I have useful service to give, I will not be deterred by the views that I held about term limits when I ran for the first time," Tancredo wrote.

Democrats were surprised by the announcement. "We'll leave it to the voters of Colorado who will cast their judgment," said Jennifer Palmieri, spokeswoman for the Democratic National Committee in Washington, D.C.

Tancredo's announcement closely mirrors that of Scott McInnis, R-Colo., who vowed in 1992 to serve only six years. He later retracted that three-term limit pledge to voters.