Donald Trump stepped up his attack on Ted Cruz on Monday, tweeting that the Texas senator needs to “either settle his problem” of having been born in Canada or leave the race.

“It’s time for Ted Cruz to either settle his problem with the FACT that he was born in Canada and was a citizen of Canada, or get out of race,” Trump tweeted Monday morning, on the heels of Fox News polls released over the weekend that showed him with double-digit leads over Cruz in both Iowa and New Hampshire.