WASHINGTON — Senator Ted Cruz was emphatic on Thursday in Nevada when he summarized his immigration stance to reporters before a rally.

“Let’s have a moment of simple clarity,” he said, responding to intensifying accusations by Senator Marco Rubio that he has changed his views on the topic. “I oppose amnesty. I oppose citizenship. I oppose legalization for illegal aliens. I always have and I always will.”

But in his first venture into presidential politics, as a policy aide on George W. Bush’s 2000 presidential campaign, Mr. Cruz, of Texas, advocated a far more equivocal answer to the question of what to do with those in the country illegally.

“Not at this time,” Mr. Cruz suggested that Mr. Bush, then the Texas governor, say if asked whether he supported “amnesty for illegal aliens already here.”