The chart above is from a recent Gallup Poll -- done between May 1st and 31st of a random national sample of 15,219 adults, with a 1 point margin of error.
Back in September of 2015, the monthly Gallup Poll showed the percentage of Americans saying they were a member of either the Republican or Democratic parties (or leaned toward one of those parties) was roughly equal -- about 42% for each. Even as late as November of 2016, the parties had only a 2 point difference -- Republicans 42% and Democrats 44%.
But since Donald Trump and the Republican Congress have taken office (and displayed their incompetence), that has changed. Their is now a 7 point difference between the parties (Republicans 38% and Democrats 45%) -- and that gap seems to be still growing (more from voters fleeing the Republican Party than new members of the Democratic Party. The Republicans have shrunk by 4 point while the Democrats have gained 1 point.
If the Republicans don't stop the bleeding and turn this trend around, the 2018 election could be disastrous for them (assuming the Democrats can get their voters to the polls).