Political scientists have observed that American politics has deteriorated into an unstable combination of weak parties and strong partisanship — dry brush for the likes of Trump and Blankenship to ignite. The 2002 McCain-Feingold campaign-finance reform restricted party fundraising, and the Citizens United Supreme Court ruling in 2010 essentially destroyed parties by giving everybody else freedom to spend unlimited sums to buy politicians. The moderating influence of parties was replaced by the radicalizing influence of dark money.
Related to this, partisanship in Washington escalated, aggravated by partisan redistricting that puts almost all House members in safe seats where the only threat comes from primaries. Primary voters tend to favor extreme candidates — who, once in Congress, turn politics into warfare.
McConnell, who championed unlimited dark money, cheered Citizens United and dramatically accelerated the partisan revenge cycle on the Senate floor, is now the victim of extremists his own actions created.