WASHINGTON -- Colorado Republican Rep. Tom Tancredo, an outspoken opponent of illegal immigration, said Tuesday he will form an exploratory committee this week for a possible presidential campaign but does not plan to give up his House seat.

After making a weekend trip to Iowa, home to the first nominating caucus in January 2008, Tancredo said voters there told him other presidential candidates don't share their views.

"They believe that there is a void in this race that none of the other candidates are willing or able to fill," he said.

Tancredo has said immigration would be a central issue if he does run, but not the only issue.

"It's a big plank in the platform but the fact is, there are a lot of issues that you have to discuss when you run for this office," Tancredo said. "It can't be just one."

Campaign spokesman Tim Haley said Tuesday Tancredo would also push opposition to abortion and other conservative themes as well.

Haley said grassroots leaders in Iowa told Tancredo they want a "true conservative" in the race to counter other potential GOP candidates, including Arizona Sen. John McCain, former Massachusetts Gov. Mitt Romney and former New York Mayor Rudy Giuliani, who are seen as moderates by some.

While in Iowa, Tancredo promoted his book, "In Mortal Danger," which warns about the need to secure U.S. borders.

Tancredo was partly testing the support among Iowa Republicans for McCain, who has proposed a guest worker program for illegal immigrants.

Tancredo and other anti-immigration activists have heavily criticized McCain for being too soft on immigration.

A bill McCain helped sponsor would have provided a path to citizenship for some of the estimated 11 million illegal immigrants in the country now. Tancredo advocates securing the border before creating a guest worker program.

Tancredo met with activists and GOP leaders in Iowa.

"Too many people believe that their leaders in Washington have forgotten them," Tancredo said after the visit. "I want to make sure that their voices are heard in the upcoming election."

Last year, Tancredo said he would campaign against fellow Republican candidates supporting a path to citizenship for illegal immigrants -- even Sen. John McCain.