des moines republican front-runner candidate donald trump big lead race 2016 presidential nomination nationally swamp opponents 40.6 percent share survey reuters-ipsos track poll find friday survey 582 respondents find trump high percentage vote next four challengers combine texas senator ted cruz draw 10.5 percent retire neurosurgeon ben carson 9.7 percent former florida governor jeb bush 9.2 percent florida senator marco rubio 7.2 percent hypothetical matchup democratic front-runner hillary clinton trump clinton hold statistically insignificant lead 41.4 percent 40.8 percent trump lead national poll republican voters months also hold lead poll early vote state iowa new hampshire iowa caucus days away feb. 1. separate reuters-ipsos poll friday find u.s. economy re-emerge major concern voters economic concern take back seat fear terrorism last year islamic state attack paris survey 1,614 respondents find terrorism top concern 20.3 percent u.s. economy close behind 18.2 percent report steve holland edit richard chang sap sponsor coverage independently produce staff reuters news agency poll trump lead 40.6 percent