The Democrats owe their success this year to independents. Independents voted 54 to 42 for Democratic congressional candidates. This is the first time since 2008 that independents voted for Democratic congressional candidates over Republican congressional candidates. Here's your history:
Independent congressional and pickup of seats.
2006 independents voted Democratic by a margin of 57-39 over Republicans with 4% voting third party. Democrats picked up 33 house seats. Democrats regained control of the house.
2008 Independents voted 51-47 for Democratic congressional candidates. Democrats picked up 21 house seats.
2010 independents voted 56-37 for Republican congressional candidates with 7% voting third party. Republican picked up 63 house seats. Republican regained control of the house.
2012 Independents voted 50-49 for Republican congressional candidates. Republicans lost 8 house seats.
2014 independents voted 52-46 for Republican congressional candidates. Republicans gained 13 house seats.
2016 Independents voted 51-47 for Republican congressional candidates. Republicans lost 6 house seats.
2018 Independents voted 54-42 for Democratic congressional candidates with 4% voting third party. Democrats picked up 37 seats with 5 seats not yet decided.
I would add this warning. If the Democratic controlled House does America's business, America as a whole and not just the Democratic base's business. They will remain in control for quite a long time. Over the last two years, the Republican house went about only Trump's business, the GOP base's business. Because of that, they got thumped.
Just keep this in mind, what independents give, they can take away. Ask the Republicans of 2006, ask the Democrats of 2010 or the Republicans of 2018. Forget political vendettas, concentrate on America as a whole. Keep in mind today Democrats make up only 30% of the total electorate, Republicans 28%, independents 40%. Here's what most of what independents want.
https://news.gallup.com/poll/220265/americans-favor-compromise-things-done-washington.aspx