Reagan eventually did endorse Ford and people forgave him for not doing it at the convention.
What you do has consequences - it gave us Jimmy Carter for four years.
If you’re going to be a team player be all in or stay home.
And yet Reagan told the convention to 'go forth united for victory in 1976' and put his arm around Ford on stage after the speech. Which I and a think most saw as an endorsement.
In Cruz's case, his words and actions are without question meant to sabotage Trump's candidacy - and may be unforgivable.