With the Supreme Court Citizens United ruling, limiting the amount of money that one can contribute to one side or the other in an election was equated to limiting free speech and organizational entities (i.e. corporations) were granted that same right. While a constitutional amendment could redress that ruling, there are other means.
Use public financing to ensure that all candidates in a campaign have access to the same amount of funding whether it be direct or indirect (Super PACs). If one individual or company donates 10 million to one side (campaign or Super PAC), and the other side only raises 5 million, campaign financing would make of the difference.
To avoid any negative impact to the debt, pay for this increased use of public financing with a tax on the campaign donations.