It's Cleveland or bust.

The only way Republicans can stop Donald Trump from capturing their party's presidential nomination will be if they go to the July party convention without a clear standard-bearer.

Virtually every GOP member of The POLITICO Caucus – a panel of operatives, activists, strategists and elected officials in four key March-voting battleground states – said after Super Tuesday, the door has closed on the possibility of another candidate winning a majority of delegates.

Insiders in these four states – the March 15 primary states of Florida, North Carolina and Ohio, plus Virginia, which voted this week – were split on whether Trump would clinch the nomination before the convention.

Half suggested a diffuse field would deny Trump a majority of delegates, while the rest said Trump was on a path to victory – and establishment efforts to wrest away the nomination on the floor of the convention hall would backfire.

"Trump has not gotten over 50 percent of current delegate allocation," said a Florida Republican, who, like all respondents, completed the survey anonymously. "The establishment will try and keep him from getting there to push for a brokered convention. It's the only way to stop him."

"Republicans need to figure out a way and rationale to fully support Trump assuming he does as well in upcoming contests as he did on Super Tuesday," countered a North Carolina Republican. "The door is swinging shut on the others and will slam shut if he wins on March 15. I'm not a Trump supporter; however, he is winning fair and square. If Republican Party tries to support some disingenuous alternative, that will have the same effect as throwing a case of hand grenades in our own tent."

An additional problem for the anti-Trump forces: Trump is well-positioned in the states leading up to the big delegate prizes on March 15. In a survey conducted after Super Tuesday but prior to Thursday night's GOP primary debate in Michigan, GOP Caucus panelists gave Trump the edge in nearly every state that will vote between now and March 8.