Many Republicans see Marco Rubio as a rising star who can help them win over the fast-growing Hispanic population, but the Florida senator says toned-down rhetoric on the hot-button issue of immigration would be more likely to bring those voters to the GOP. "The policies are important, but the rhetoric is sometimes the impediment," Mr. Rubio said in an interview. "Sometimes—and I'm not pointing fingers at anyone—the way the message is communicated is harmful and has hurt Republicans."