Selzer poll of Iowa: Cruz 25, Trump 22, Rubio 12, Carson 11

Cruz has the support of 25 percent of those likely to attend the Republican caucuses on Feb. 1, closely followed by the billionaire real estate mogul at 22 percent. The poll indicates that while Trump’s supporters are more committed, the firebrand junior senator from Texas appears to have more room to grow his support.

Still, Trump’s ability to close the 10-point gap that Cruz had opened up over him a month ago is more due to Cruz losing altitude than Trump gaining it: Trump is up 1 percentage point since the poll was last taken in early December; Cruz has dropped 6 points during a period when he was under almost constant attack from Trump and other rivals, as well as Iowa’s powerful corn-based ethanol industry. In the last two weeks, Trump has aired 2,324 ads in Iowa, far more than any of his rivals, according to data compiled by the media tracking company Kantar/CMAG.

The race for third place in Iowa is equally tight, with Rubio distancing himself from other establishment candidates at 12 percent, closely followed by Carson at 11 percent…

Trump’s most recent line of attack against Cruz—questioning his legal eligibility for the presidency because he was born in Canada to an American mother—doesn’t appear to have much punch. Just 15 percent of likely Republican caucus participants say they’re bothered that Cruz was born outside the U.S., with about half of those being Trump’s own supporters.