The Supreme Court issued a summary opinion overturning Montana’s restrictions on corporate spending on electoral speech. The court did so 5-4 along the lines you might expect. As John Samples points out in this short video, the court’s opinion in Citizens United that “political speech does not lose First Amendment protection simply because its source is a corporation” is not as settled as fans of freewheeling political communication might hope.
To overturn Citizens United, lawmakers might pursue a Constitutional amendment that would allow Congress to prohibit funding of electoral speech. But as Samples notes, a number of proposed amendments go far beyond banning corporate political speech and simply outlaw all private spending to influence elections. One should presume that such a ban could include Sharpies and posterboard for your standard-issue homemade political sign or the purchase of certain Shepard Fairey prints.