Ted Cruz has opened up a firm lead over rival Donald Trump in first-in-the-nation state Iowa, 31 to 21, according to a new Bloomberg Politics/Des Moines Register poll released Saturday.

The poll is closely watched by political analysts who consider it a reliable measure of Iowa caucus voters.

The poll is grim news for former Iowa front-runner Ben Carson, who drew just 13 percent support, less than half the 28 percent support he enjoyed in October.

Marco Rubio comes in fourth, 10 percent, followed by his former ally, Jeb Bush, at 6 percent.

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