As this "Poll of all Polls aggregate graph to March 29th from Huffington Post Pollster shows Ted Cruz (and Kasich) where both in a sharply ascending mode after all the other competitors, especially Marco Rubio had dropped out.
Donald Trump had peaked and his trend line had commenced a downwards turn. This was an absolutely crucial time in the campaign for Cruz with his competitors mostly gone and Wisconsin coming up for him ahead of the less promising eastern states.
Then on April 2nd the graph for Trump turned upwards in a sharply accelerating line and gained almost ten points from 43.2% to 53.1% in the latest May 1st poll where the campaign ended
What can be the reason for this striking change for Trump and decline for Cruz at what was a crucial time for both? The answer seems, clearly to be the public's perception of the manifestly unfair 'stealing of delegates at the Colorado Caucuses which commenced, hardly coincidentally on April 2nd and ran, in the glare of nationwide publicity, to April 9th: