"The Senate Democrats Schumer leads are an ideologically polyglot group confronting tough political terrain. In the 2018 midterms, 25 of their caucus members face reelection. Ten represent states Trump carried, five of them deeply red."

A gain of ten (10) Senators would be a 61 Senator majority for the Republicans.  Any gain above that would just be icing on the cake.  The House of Representatives is expected to maintain its Republican majority in 2018. 

I'm not too sure how Dionne expects the Democrats  to be a 'barrier' to Trump now or then,