The spots hitting Rubio were run by three different super PACs. Two, Keep the Promise I and Courageous Conservatives, support Sen. Ted Cruz of Texas. The other, Right to Rise USA, is backing former Gov. Jeb Bush of Florida. There are likely much more in negative ads on the air, or in the pipeline, than has been reported, as earlier this month Right to Rise USA announced a $3 million television ad campaign against Rubio.

Meanwhile, no negative super PAC spots were reported as targeted Trump, the undisputed front-runner nationally and in New Hampshire. For that matter, only a minimal amount of critical ads were directed at Cruz, who is running second in most polls and tied for first with Trump in Iowa. Pursuing America’s Greatness, a super PAC supporting long shot candidate Mike Huckabee, who won the Iowa caucuses in 2008, is the only one to target Cruz.

Trump and Cruz aren’t likely to tumble from the top without pressure.

It’s going to take negative advertising. Some Republican strategists believe it’s possible that the intensifying rivalry between Cruz and Trump in the battle Iowa caucus-goers could depress their high numbers ease pressure on Rubio, and possibly other candidates. That this hasn’t happened yet is atypical, but could be because Trump is viewed as a weaker candidate if the primary goes deep into the spring than is Rubio.