Bloomberg that according to James Carville (my emphasis):
There’s a test coming for whether the grass-roots outpouring of the past 40 days is translating into voting power. That’ll be a special election set for April 18 for the suburban Atlanta congressional seat of Tom Price, who left the House of Representatives after being named Secretary of Health and Human Services. The district hasn't elected a Democrat in more than four decades, and Price won last November by almost 25 percentage points. But Trump beat Hillary Clinton in his district by less than two percentage points and Democrats are pouring money and energy into the contest.
"This is the kind of Republican district we have to win if Democrats are going to do well in 2018," Carville said, predicting that an upset victory would send shock waves through Republican congressional ranks.