Numbers USA, an influential organization that champions stricter immigration laws, is downgrading its ratings of Donald Trump after the GOP front-runner wobbled on his immigration stance at the Republican presidential debate on Thursday night. The grass-roots group was displeased with Trump’s comments suggesting that there is a shortage of U.S. workers for both high-skilled industries and for seasonal jobs, such as those for hospitality employees at Mar-a-Lago Club, Trump’s luxury resort in Florida. In a statement, Roy Beck, the group’s president, called Trump’s remarks “disconcerting” and said Numbers USA was downgrading Trump to a “B+” grade from an “A-” as...