WASHINGTON (AP) — President Donald Trump has been an unpredictable ally for Republicans.

After he announced plans to go to Indiana Wednesday to sell the GOP tax overhaul plan, 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 GOP's high 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.

The dysfunctional dynamic now stands at the center of Republicans' last chance this year to prove they can pass major legislation — a sweeping, multitrillion-dollar tax cut that GOP officials believe is crucial to the party maintaining its congressional majority in the midterms.