Democrats have a chance to retake the Senate—a proposition that seemed far-fetched earlier this year. But instead of unifying ahead of Election Day, the party is already engaged in infighting and finger-pointing. Allies of the party’s leaders privately say that an insurgent left-wing has unnecessarily sucked time and money into races that should have been easier for Democrats, to the detriment of other candidates. Progressives say the party’s establishment isn’t doing everything possible to help them win, according to people familiar with the conversations. Tensions spilled out publicly Sunday when a Democratic congresswoman in Michigan, who lost this year’s Senate…