President Trump and his aides are privately worried that impeachment investigations will stifle administration work, such as efforts to procure a North American trade deal, even as they publicly gird for a fight, according to multiple insiders. But operatives on his reelection campaign see an opportunity to mobilize the base, setting up a divide between them and White House aides trying to head off damage. Trump’s confidants charge that the Democrats are acting in bad faith and trying to stymie the president’s chances to advance some policies before the campaign shoves everything else aside. Trump himself unsuccessfully tried to prevent...