DENVER, Colo.—It appears "Romnesia"—the term President Barack Obama coined last week at a campaign stop in Virginia to criticize Mitt Romney for shifting positions—is here to stay.

Sure, it's cute. The crowds love it. And since Friday, Obama has not delivered a campaign speech without mentioning it. But Obama and his team aren't laughing. Romnesia, they say, isn't a trivial idea. It's the most important issue of this presidential campaign.

"On Romnesia, this is about as big a thing you can have in a presidential election. It's about trust," White House senior adviser David Plouffe told reporters aboard Obama's press bus Wednesday. "You cannot trust Governor Romney."

Trust has become the driving theme of Obama's campaign in the final weeks before Election Day. Obama may have been careful about appearing too aggressive against Romney earlier in the fight, but ever since the presidential final debate, he has made his opinion clear: He not only disagrees with Romney on issues of policy, he's not afraid to say he thinks Romney has an integrity problem.

"We joke about Romnesia, but all this stuff leads to something that is essential to your choice," Obama told a crowd of about 16,000 people who rallied for him at Denver's City Park Wednesday. "Trust."

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If the campaign finds the strategy to be effective, and it appears that so far it does, the trust theme could last until Election day.

"You have never seen a major party nominee this close to an election try and fudge what he's going to do as president," Plouffe said. "We want to make sure people understand the differences here. Governor Romney is someone in the closing of his campaign who has been dishonest about the position he intends to try and execute as president."