Joe Biden's vow to phase out the oil industry at Thursday's debate creates a problem for Democratic candidates in red-leaning states and swing House districts as it gives Republicans an opening to tie them to their party’s left wing. Senate Majority Leader Mitch McConnell (R-Ky.) said in April of last year that he wanted to make the 2020 election “a referendum on socialism.” That strategy ran into a big problem when Democrats nominated Biden, a candidate who developed a reputation as a moderate during his decades in Washington, for president. Now Biden’s blunt affirmation that “yes” he “would transition” when...