In 2018 when Trump was not on the ballot, 538 calculated that 15 out of a total of 17 candidates he endorsed won open-seat primary contests that often pit two conservatives against each other. Trump's win-loss ratio was better than 11 conservative and business groups, including the Chamber of Commerce, the Club for Growth and the Koch network. There is a recent precedent for the current Trump-McConnell conflict that I just mentioned, the Tea Party versus the Republican establishment in the elections of 2010 and 2012. The results were a mixed bag. One group of Tea Party candidates won primaries but lost the general election in states as diverse as Delaware, Colorado, Nevada, Alaska and Indiana. Another group,…