WASHINGTON — Tom Steyer, the billionaire environmentalist and Democratic political donor, announced Monday that his advocacy group, NextGen America, will spend $30 million to help Democrats retake the House in 2018, with the aim of eventually impeaching President Trump. Mr. Steyer also announced that he would not run for Senate or governor in California, as had been speculated. Mr. Steyer, who was a major donor to President Barack Obama’s 2012 campaign, emerged as the largest single political donor of the 2014 midterm election. His group spent $74 million to back Democrats that year, and $87 million in the 2016 elections....