While the Left often claims to be champions of civil rights, but often vehemently oppose free speech rights. One class of speech the Left often tries to suppress is political speech. They support this form of political oppression on the grounds that money spent on speech corrupts politicians. By contributing to a politicians political campaign, supposedly if the politician wins he will reward the donor. This may or may not be true, but limiting people's ability to support their candidate will not stop politicians from being corrupted.
One way that people voice their support or opposition to a candidate is by coming together as part of a corporation. The Left claims silencing the views of these corporations does not violate anyone's right because corporations are not people. But corporations are people. They came together to use a corporation as a means of jointly expressing their political views. These are the people the anti-free speech Left wants to suppress.
The case that brought this issue to the forefront was Citizen's United vs FEC. Citizen's Unitedwas a corporation, a group of people, who produced a video opposing Hillary Clinton. They wanted to run ads for this film during 2008 Democratic primaries. This was in violation of the the McCain–Feingold Act which attempts to silence such political dissent.
These types of laws have the clear purpose of silencing speech. This is not about if a corporation is a new independent person. The intent of the laws is irrelevant as the ends do not justify the means
The Left supports giving the government grand powers to take from one group and give to others. To change the basic rules governing the economy, to set regulations that benefit one group at the expense of other. These powers are what are corrupting politicians not the contributions of donors. As long as they refuse to accept that the government they want will always be corrupt, they will not deal with the real problem.