Campaigns are how candidates introduce themselves and their ideas to the electorate.   Any attempts to limit how much a person chooses to donate is a violation of that donors free speech rights.  A person has a right to make a movie, write a book, advocate, volunteer or donate to any cause or person they so choose.  Laws that limit free speech violate the constitution and have resulted in the criminalization of political activity.  The Supreme court is about to rule that these laws are vague and too easily abused by the Justice Department and the administration to silence opposing views.