Write In: Claire McCaskill. Like Donnelly, she ran too far to the right and neglected Democratic base voters. Turnout in Democratic strongholds was not as strong compared to rural areas as it had been in McCaskill's two previous Senate campaigns, and her pandering toward rural Trump voters didn't do her any favors in rural MO (I'm not saying rural outreach isn't important in MO, I'm saying that she didn't use the correct approach in doing so). Remember that MO voters passed ballot measures to increase the minimum wage, legalize medicinal marijuana, repeal right-to-work, and reform campaign finance, so had she emphasized her support for these issues more, she would almost certainly have done better (and might have held on to her seat).
In particular, given how unpopular the Republican healthcare efforts were, the red-state Democratic Senators could have benefited if they prided themselves on being the deciding vote against it and made a big thing of warning that if they lost their seats the bill would be bought back. Tbf, Democrats did campaign a lot on healthcare anyway and McCaskill, Donnelly and Heitkamp probably were doomed regardless in retrospect, but perhaps they could have gotten a few more votes.
