DES MOINES — The Republican presidential field banded together on Friday against the man they now see as their common enemy, Senator Ted Cruz of Texas, whose perceived front-runner status before the Iowa caucuses appeared to grow even more uncertain just three days before the voting begins.

As candidates spent the day attacking the senator — some of them clearly giddy over being free to unleash on a rival they have never had much affection for — Mr. Cruz responded by sharpening his own attack strategy. He shifted nearly all his negative advertising here from going after Donald J. Trump, who has overtaken him in recent polls, to Senator Marco Rubio of Florida.

The two freshman senators are locked in a race that is tightening by the day here, with Mr. Rubio gaining on Mr. Cruz in both public polling and internal surveys conducted by both campaigns.