Newt Gingrich, a former House of Representatives speaker, gave a composed performance in the first debate since he surged to the top of polls. He backed an overhaul in immigration policy that would include a guest-worker program similar to plans condemned by conservatives in the past.

The debate, the second on foreign policy in the last 10 days, featured sharp exchanges on a broad range of issues, including anti-terrorism laws, Pakistan and Afghanistan.

The Republicans ganged up on Pakistan and questioned whether the US could trust it. Texas Governor Rick Perry called Pakistan unworthy of US aid because it had not done enough to help fight al Qaeda.

Representative Michele Bachmann, a member of the House Intelligence Committee, called Perry "highly naive" and said the US should demand more from a "violent and unstable" Pakistan with nuclear weapons.

Former Massachusetts Governor Mitt Romney said US aid to Islamabad could help "bring Pakistan into the 21st century, or the 20th century for that matter."

Democrats have criticized the Republican field for a lack of foreign policy knowledge, but the wide-ranging debate was substantive and devoid of major missteps. Jon Huntsman, a former US ambassador to China, is the only candidate with deep foreign policy experience.

In a discussion of ways to halt Iran's nuclear weapons program, Perry and Gingrich backed sanctions on the Iranian central bank. Gingrich called it "a good idea."

Eight Republicans participated in the debate at Washington's DAR Constitution Hall, which aired live on CNN and was the 11th for Republicans seeking the right to challenge President Barack Obama in 2012.

Gingrich, as the latest in a series of conservatives to challenge the more moderate Romney for the top spot in the Republican race, had the most to lose in the showdown.