NEW YORK:-US Republican presidential candidates Senators Ted Cruz and Marco Rubio are on the upswing as front-runner Donald Trump sees his vote base shrink, according to the first national poll after the Iowa caucuses. The Democratic firm Public Policy Polling survey which was released Thursday, found that 25 percent of the Republican-primary voters were still supporting Trump while Rubio and Cruz both ranked in second at 21 percent.Former neurosurgeon Ben Carson was third at 11 percent, while none of the remaining Republican candidates took more than 5 percent of the vote.–SPECIAL CORRESPONDENT