Sanders is addressing supporters in Keene, N.H.
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"My experience in Iowa was a great one," he tweeted.
Writers in The Washington Post and National Review also took Trump to task.
The graffiti was quickly removed.
His rally in Cedar Rapids is scheduled for 2:30 p.m.
Republican presidential front-runner Donald Trump launches his final pitch before tonight’s Iowa caucuses at the Ramada Waterloo Hotel and Convention Center in Waterloo, Iowa.
"You guys don't always keep your promising, evidently," Palin told NBC.
VIDEO | More than 50 people were killed in bombings outside Damascus.
"I believe 2016 will be a national security election," John Bolton said.
Police reported 2 have been shot, 1 stabbed.