Tim Pawlenty

Trump, an uneven ally for GOP, tests his influence on taxes

When President Donald Trump announced plans to go to Indiana Wednesday to sell the GOP tax overhaul plan, party leaders cheered his engagement on the high-stakes issue. When the White House said one of Trump's traveling companions would be Indiana's Democratic Sen. Joe Donnelly — a top target in next year's midterm elections — the hopes turned to head-smacking disbelief.

The episode underscores the extent to which Trump has been an uneven — or at best, unpredictable — ally for his party, both on policy and politics. Time and again, Trump has complained about his party as loudly as he vents about Democrats, or shifted his position as Republicans leaned on him for support. The...