According to the Federal Election Commission's review of recent disclosure reports from Sen. Bernie Sanders’ presidential campaign, theSanders campaign has accepted thousands of illegal contributions: 

The Federal Election Commission sent a letter to the Democratic presidential candidate’s campaign committee . . . with a 90-page spreadsheet listing 3,457 “excessive, prohibited, and impermissible contributions.” 

The campaign’s January financial disclosure filing listed contributions from foreign nationals and unregistered political committees, the FEC said. Other contributions came from donors who exceeded the $2,700 per-election limit. 

[. . .] The FEC sent a letter to the Sanders campaign earlier this month flagging an additional 1,316 “excessive, prohibited, and impermissible contributions” in the fourth quarter of 2015.

Scott Blackburn from the Center for Competitive Politics points out that the illegal contributions to the Sanders campaign are demonstrative ofpervasive problems with the campaign finance system:

Such letters are routine in political campaigns; it is essentially impossible to follow the FEC’s reporting requirements to the letter of the law, and even more difficult to make sure that donors don’t accidentally violate federal contribution limits in their zeal to support their preferred candidate. So the regulators at the FEC send letters to warn campaigns that if they don’t fix their paperwork, the Commission is going to audit and fine them. 

These violations, in relation to Sanders, whose candidacy has been propelled by the passion of hundreds of thousands of small donors, provide a window into how ridiculous our campaign finance regulatory regime, particularly our strict campaign contribution limits, have become.

While Sen. Sanders may criticize the campaign finance system, his campaign still must abide by the current law.  The failure of his campaign to follow some of the relatively simple regulatory requirements of the campaign finance system should make him realize that the true problem with the campaign finance system is the burden it places on the exercise of First Amendment freedoms, not “dark money.”