The Republicans blocked the amendment to overturn Citizens United because they know they are the big beneficiaries of large donations by rich individuals and corporations. Most of that money is spent supporting GOP candidates. But the Democrats are fighting back with a secret weapon of their own -- small donors (those giving $200 or less).
On the national level, the Democratic Congressional Campaign Committee (DCCC) has raised $41 million more than the National Republican Campaign Committee (NRCC) in donations from small donors. And the same thing is true in individual congressional races across the country -- both in blue and in red states. The average Democratic incumbent or hopeful has gotten $179,300 from small donors, while the average Republican incumbent or hopeful has gotten only $78,535 in small donor money (a difference of more than $100,000).
The National Journal credits this to the more effective use of the internet by Democrats, and that is probably true to a large extent. But I would like to believe that ordinary Americans (those who can afford only a small donation between $1 and $200) just prefer Democrats to Republicans. I guess we'll find out on election day if that is true.