Hillary Clinton speaks at a town hall in Fairfield, Iowa, on Dec. 22, 2015. (Melina Mara/The Washington Post)

DES MOINES — Hillary Clinton was Zen-like about nemesis Donald Trump on Monday, shrugging off a question about one of the Republican presidential front-runner's attacks on her by implying that some of what he says is divorced from reality.

"I've adopted a New Year's resolution," Clinton said at a question-and-answer session with voters in Cedar Rapids, Iowa.

"I'm going to let him live in his alternate reality," she said. "I'm not going to respond."

Clinton had been asked to respond to what the questioner said was Trump's charge that Clinton and President Obama had "created" the Islamic State militant group through their actions in the Middle East.

Clinton made a face and adopted a flat, all-business tone to deliver her answer.

The policy clearly isn't across the board but appears to be part of the candidate's effort to set herself apart from the  billionaire, known for his sharp and often personal attacks against her. Her campaign has frequently suggested that point-by-point responses to what Clinton calls Trump's "bluster" and bullying is beneath her.

A top aide, Christina Reynolds, said last week that Clinton will take on Trump when he attacks vulnerable groups, including immigrants.

Earlier Monday in Davenport, Clinton delivered remarks but did not take questions. There, she had referred to some of Trump's policies but did not discuss him by name.