Keller @ Large: Obamacare A Major Issue In 2016 Election

BOSTON (CBS) - Like any rule, it is made to be broken.

But a good rule of thumb about elections involving an incumbent is that they are in large part a referendum on that incumbent, and whether their ideas and performance deserve to be renewed.

President Obama won't be on the ballot this fall, but it was clear in Thursday night's Democratic debate that the centerpiece of his administration – the Affordable Care Act – is a major issue in the race to succeed him. And if Hillary Clinton has her way, that dynamic will extend well beyond Obamacare.

Right off the bat, she and Bernie Sanders tangled over that topic, which Sanders proposes to replace with a single-payer system, or as he calls it, Medicare for all. Sanders claims it would cost the average American an extra $500 on their tax bill, but would return $5,000 worth of benefit; Clinton insists his numbers "don't add up," and suggests re-opening the Obamacare debate could roll back its gains.

But an even more telling exchange came when Clinton responded to the familiar questions about the Wall Street campaign donations she's pocketed by noting that Obama took more money from Wall Street than any Democratic candidate in history without compromising his ability or willingness to impose strict new regulations, a degree of independence Clinton claimed she, too, would retain.

Sanders called that "an insult" to the voters' intelligence.

His argument buried hers in New Hampshire, no doubt. But you wonder how it plays in South Carolina, where close to 30% of the Democratic electorate is African-American and strongly pro-Obama.

Between these two, and the Obama-phobia of the GOP field, something tells me this Obama-less election could nonetheless be another one of those referendums on the incumbent.

Listen to Jon's commentary: