WASHINGTON — President Joe Biden’s recent political revitalization hasn’t stopped some Democrats in competitive midterm races from treating him like an in-law: always welcome but seldom wanted. Democratic candidates are tiptoeing around his official visits, making case-by-case decisions about whether to appear with him. They're also speaking out against him on certain issues. One campaign outright said it wasn't interested in a Biden visit, while a veteran House Democrat told voters she is “fighting back” against the president. Democrats are walking a tightrope in characterizing their ties to Biden, which might seem counterintuitive considering the streak of legislative victories that...