Story highlights Trump leads Sen. Ted Cruz, R-Texas, who is in second place in the GOP race, among likely Republican caucus-goers, 37% to 26%

Sanders, meanwhile, has opened up an eight-point lead over Democratic rival Hillary Clinton, leading her in Iowa 51% to 43%

Washington (CNN) Donald Trump holds a commanding lead in Iowa as Sen. Bernie Sanders takes control of the Democratic race in the critical first-in-the-nation voting state, according to a new CNN/ORC poll released Thursday.

Trump leads Sen. Ted Cruz, R-Texas, who is in second place in the GOP race, among likely Republican caucus-goers, 37% to 26%. Sen. Marco Rubio, R-Florida, is in third at 14%, the only other Republican in double digits. Ben Carson failed to register half of Rubio's support and is in fourth place at 6%.

Sanders, meanwhile, has opened up an eight-point lead over Democratic rival Hillary Clinton, leading her in Iowa 51% to 43% among likely Democratic presidential caucus-goers.

The sampling is key for both leaders: Only including voters who previously caucused in their party's most recent competitive caucus, Cruz is neck-and-neck with Trump, with 30% for Cruz to 28% for Trump. Rubio is steady at 16% in that sample.

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