President Trump is increasingly directing his frustrations at specific Senate Republicans, including turning his fire on key allies in recent days. The president this week publicly excoriated Sens. Chuck Grassley (R-Iowa) and Josh Hawley (R-Mo.) over upholding the “blue slip” tradition and pushing a congressional stock trading ban, respectively. While Trump has long had a tendency of airing his grievances out in public — especially toward those he views as disloyal — the latest barbs about staunch backers are raising eyebrows and drawing questions about whether the strategy will continue to be effective in advancing his agenda. “I don’t think...