Donald Trump is now more likely than not to win the Republican nomination.

On the day of the Iowa caucuses, the political prediction markets, where traders bet on the outcome of the election, give him a 51 percent chance of becoming the G.O.P. nominee.

Although the Republican field still nominally contains an unusually large slate of candidates, prediction market traders see a four-horse race. Mr. Trump is the clear favorite. The second favorite is Marco Rubio, at 33 percent.

Ted Cruz, who appears to be Mr. Trump’s most serious rival to win in Iowa, is not expected to do as well in less conservative states, and so he is given only an 8 percent chance. The early front-runner in the race, Jeb Bush, is now given only a 6 percent chance.