WASHINGTON -- A day after receiving the Presidential Medal of Freedom, retired Justice John Paul Stevens on Wednesday night backed President Barack Obama's suggestion during his 2010 State of the Union address that the Citizens United decision could lead to "foreign entities" bankrolling American elections.

He urged the U.S. Supreme Court to explicitly explain why the president's words were "not true," as Justice Samuel Alito famously mouthed on camera, breaking the justices' usual stoic appearance during the president's annual speech.

Stevens has been a trenchant critic of Citizens United since the court decided the case in January 2010. On the day the opinion was announced, he spent 20 minutes reading from the bench a summary of his 90-page dissent. Stumbling over some words that day convinced Stevens, now 92, to retire, but he continued to condemn the ruling in speeches, writings and even on the Colbert Report.

In a speech at the University of Arkansas' Clinton School of Public Service, Stevens challenged his former colleagues to defend Alito's "not true" moment by reconciling the court's sweeping language in Citizens United that the First Amendment "generally prohibits the suppression of political speech based on the speaker's identity," with its subsequent decision -- made without briefing, argument, or written opinion -- to uphold a ban on campaign spending by non-citizens.

Alito's reaction, Stevens said, "persuade[s] me that that in due course it will be necessary for the court to issue an opinion explicitly crafting an exception that will create a crack in the foundation of the Citizens United majority opinion." In doing so, he continued, "it will be necessary to explain why the First Amendment provides greater protection to the campaign speech of some non-voters" -- that is, domestic corporations -- "than to that of other non-voters" such as the Canadian Harvard Law School graduate who remains barred from making campaign contributions.

The lawsuit brought by the Canadian citizen "unquestionably provided the court with an appropriate opportunity to explain why the president had misinterpreted the Court's opinion in Citizens United. "[T]he court instead took the surprising action of simply affirming the district court without comment and without dissent."

The decision in that case, Bluman v. FEC, meant that "notwithstanding the broad language used by the majority in Citizens United, it is now settled, albeit unexplained, that the identity of some speakers may provide a legally acceptable basis for restricting speech," Stevens said.

But the court cannot forever evade a written reckoning with the logical conclusion of its Citizens United decision, Stevens said.

"I think it is likely that when the court begins to spell out which categories of non-voters should receive the same protections as the not-for-profit Citizens United advocacy group, it will not only exclude terrorist organizations and foreign agents, but also all corporations owned or controlled by non-citizens, and possibly even those in which non-citizens have a substantial interest," Stevens said, referencing a case in which he joined the Citizens United majority to hold that speech made or funded by terrorist groups have no First Amendment protection. "Where that line will actually be drawn will depend on an exercise of judgment by the majority of members of the court, rather than on any proposition of law identified in the Citizens United majority opinion."

The justices will soon have another opportunity to clarify the scope of Citizens United in a challenge to Montana's corporate spending limits brought by an out-of-state organization. Although Stevens did not explicitly reference this case, which the justices will discuss in private at their June 14 conference, he lambasted the Citizens United majority for overruling a precedent that allowed states to bar corporate spending from beyond their borders. For the states with such laws, "those corporate non-voters were comparable to the non-voting foreign corporations that concerned President Obama when he criticized the Citizens United majority opinion," Stevens said.

"If the First Amendment does not protect the right of a graduate of Harvard Law School to spend his own money to support the candidate of his choice simply because his Canadian citizenship deprives him of the right to participate in our elections, the fact that corporations may be owned or controlled by Canadians -- indeed, in my judgment, the fact that corporations have no right to vote -- should give Congress the power to exclude them from direct participation in the electoral process," Stevens concluded.