Republican insiders overwhelmingly believe this summer's national convention will require multiple ballots to select the presidential nominee.

That's according to The POLITICO Caucus – a panel of operatives, activists and strategists in 10 key battleground states – with roughly 90 percent of respondents saying neither Donald Trump nor Ted Cruz will win the nomination on the first ballot in Cleveland.

It's a marked shift from a month ago, when just half of insiders were predicting a contested convention. Since that time, Trump has romped to victory in states that awarded all its delegates to the winner, like Florida and Arizona. But Cruz bounced back this week in Wisconsin – and is also dominating in states like North Dakota, Colorado and Wyoming, where party insiders pick the delegates, not rank-and-file voters.

Cruz's victory in Wisconsin, where he won 36 of the 42 delegates at stake, narrows Trump's path to the nomination. Trump's path is also impaired by his precipitous fall in national polling, which hurts the New York real-estate tycoon's standing among both Republican voters and convention delegates who want to nominate a strong general-election candidate.

"Donald Trump has one chance to win the nomination, and that is on the first ballot. Right now, I put his chances at about 40 percent, and that will require him to get some number of delegates from the unpledged delegates in states like North Dakota, Colorado, Pennsylvania, and a few territories," said an Iowa Republican, who, like all respondents, completed the survey anonymously.

But while that Iowa Republican sees Trump performing well in upcoming primaries in New York, Connecticut, Pennsylvania and other Northeastern states later this month, the Republican warned that the map is less favorable in May.