With the Republican field reshaped by the suspension of Herman Cain’s campaign, six Republican presidential candidates tried to show their conservative credentials during a policy-heavy forum shown on Fox News Channel on Saturday night.

The forum came as a new poll in Iowa showed Newt Gingrich, the former House speaker, leading the pack in that state with just more than four weeks to go before the voting there. And this week will be a busy one for the candidates, with a forum before Jewish leaders and a debate Saturday hosted by ABC News in Des Moines. Hanging over the field is the question of whether Mr. Gingrich can seize on his ascent in the polls and possibly pick up some of Mr. Cain’s conservative support.

Saturday’s forum had a gantlet of three conservative state attorneys general peppering the candidates with questions. Mr. Gingrich, the former House speaker, was pressed on how conservatives can “trust that a President Gingrich will not advance these sorts of big government approaches” that he had advocated, including his one-time support for a mandate that citizens obtain health insurance. Mr. Gingrich noted that he did so in league with other conservatives and that “every conservative has in fact left that kind of a model.”

Representative Michele Bachmann of Minnesota was asked how she would carry out her call to remove all illegal immigrants living in the United States or pay an estimated cost of $135 billion to do so. “It would be enforcement both at the border but also by the ICE agents,” she said.