WI:Democrats win the 1918 Senate elections?

OK after starting to read the ASB TL Damsels and Dirigibles by Color-Copycat, I started to wonder about American entry into the League of Nations. What if the Democrats had won the midterm elections in 1918? Would a Democratic majority lead to the Senate passing the Treaty of Versailles and allowing American entry into the League of Nations? If so what would this mean for history? Would the United State being a member state of the League keep the country from backsliding into isolationism? Would this lead to the possibility of the Democrats retaining the White House in 1920? Would the US be given its own League Mandate, similar to the British and French ones? Would the League still become a complete joke or would the US be able to give the League some teeth? Or would this change nothing?
 
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