A long-simmering debate over immigration in the Republican presidential contest boiled over on Thursday, as Senator Ted Cruz accused his colleague, Senator Marco Rubio, of “trying to jam this amnesty down the American people’s throats.”

From appearances on conservative talk radio to the campaign trail in South Carolina and New Hampshire, five Republican candidates jumped on the issue as they sought to appeal to grass-roots conservatives alarmed about immigration. And the eruption of open warfare between Mr. Rubio and Mr. Cruz, two candidates representing different wings of the party, showed the likely contours of the Republican presidential fight as it hurtles toward the Iowa caucuses in less than 90 days.

Immigration is a key vulnerability for Mr. Rubio, who was initially part of an effort in 2013 to formulate a bipartisan bill in the Senate to overhaul the immigration system. His subsequent moves to distance himself from his role in fashioning the bill have left some conservatives distrustful of his candidacy.

Mr. Cruz, in an appearance with the conservative radio host Laura Ingraham, denounced Mr. Rubio without using his name and by using a biblical allusion, saying: “You know where someone is based on their actions. As the Scripture says, you shall know them based on their fruits.”