Story highlights The Cruz campaign tried to get Rubio on board

Kasich tried to broach the subject with Rubio as well

Washington (CNN) Republican officials who oppose Donald Trump as their party nominee have spent the last few days -- ever since the withdrawal from the race of Sen. Ted Cruz and Ohio Gov. John Kasich -- wondering if there is anything they could have done to prevent this outcome.

Top officials of the Cruz campaign are convinced there is one specific step that could have stopped Trump -- and they blame Sen. Marco Rubio for not taking that step.

In early March, it became clear that Trump was well on his way to the nomination and would even likely defeat Rubio in his home state of Florida's March 15 primary. According to several sources close to Cruz, the Cruz campaign conducted several secret polls to see what the impact would be if Rubio joined Cruz as his running mate, with Cruz at the top of the ticket.

Politico reported in March that Rubio rejected the idea of a "unity ticket." But the sources close to Cruz and Rubio are now offering a much fuller picture of the extent of Cruz's polling, the reasons why Rubio said no, and the resentment the Cruz people have about Rubio's rejection of the idea.

The Cruz campaign polled in three March 15 primary states, Illinois, Missouri and North Carolina -- though not in Ohio, home to Kasich, or in Florida.

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