The national impact of gerrymandering is still debated by experts. Some see it as a major factor in Republicans' firm grip on the House, even in years when Democrats have won more total votes. Others say that natural trends have led to Democrats clustering in cities like Philadelphia and Pittsburgh while Republicans in the suburbs and exurbs have spread out more efficiently when it comes to winning House seats. And they say that if both parties' gerrymanders were eliminated, the net change would be relatively small.