Congressional Republicans have begun discussing whether they may have to vote to block President Trump’s planned new tariffs on Mexico, potentially igniting a second standoff this year over Trump’s use of executive powers to circumvent Congress, people familiar with the talks said. The vote, which would be the GOP’s most dramatic act of defiance since Trump took office, could also have the effect of blocking billions of dollars in border wall funding that the president had announced in February when he declared a national emergency at the southern border, said the people, who spoke on the condition of anonymity because...