The wailing and gnashing of teeth if the GOP manages to hold onto the House would be awesome. Because it will mean that a majority of voters actually like where the country has gone under Trump, and voting GOP wouldn't just be a vote against Hillary like it was in 2016. It would further damage the left's perception that the tides of history must flow in their direction by right.
Also, it likely would radicalize them even further, making the political landscape even more favorable for the GOP in 2020.