The chart above, from the Pew Research Center, shows the number of voters in both the Democratic and Republican primaries that have been held so far this year compared to the number of voters in those primaries in 2014.
In 2014, the Republicans outvoted Democrats in those primaries by about 1.2 million votes. That translated into a very good year for the Republicans. This year is vastly different. In 2018, the Democrats have outvoted the Republicans by about 2.9 million votes. That shows the Democrats are very enthused to vote this year.
The unknown factor is the Independent vote -- the voters who didn't vote in either party's primary, but will vote in November. Will they turn out in large numbers, and if they do, which way will they break? The only hopes for Republicans to stave off a "blue wave" this year is for either Democrats to stay home in November (not likely seeing the enthusiasm they have) or for Independents to break big for Republicans. I think it is unlikely the Independents will give the GOP the support it needs -- especially considering the poor view they have of both Trump and the GOP-controlled Congress.
