Trump's probability of winning the nomination fell to 44 percent on Friday from 67 percent a week ago, according to the website, which is run by Victoria University in Wellington, New Zealand. For Republican rival Ted Cruz, a U.S. Senator from Texas, it rose to 34 percent from 14 percent during the same period.
The probability that the Republicans will have a brokered convention to decide the nominee for the Nov. 8 election jumped to 69 percent from 43 percent a week ago, according to PredictIt, an online predictions market where users place money on who they think will win the election. Many users are from the United States.
Celebrity businessman Trump is facing pressure to take a more serious approach to his presidential campaign from supporters worried that a string of recent missteps and controversial comments may do lasting damage.
A Cruz win in Tuesday's primary in Wisconsin would make it harder for Trump to reach the 1,237 delegates needed to clinch the nomination before the Republican national convention in July.
If Trump does not win enough delegates, then the party faces
the prospect of holding a contested, or brokered, convention in which party leaders try to negotiate nominating a compromise candidate.