Rather obvious if looking at some of his history:
“I have been wondering why Lawrence Tribe spoke up about this topic.” “Rather obvious if looking at some of his history...”
Consider this as one motive of Tribe - he was Al Gore’s lawyer when Gore was trying to take the election from George Bush - Tribe lost. He would love to take down a Republican presidential candidate to assuage his failure to put Gore in the White House. Taking down Cruz would cause Tribe to celebrate by partaking in numerous glasses of an adult beverage.
Tribe and other liberals think Hillary would beat Trump, so make Trump the candidate, not Cruz whose numbers say he would beat Hillary. If Tribe could take out Cruz, he could take the credit for Hillary becoming president. After all these years when Tribe lost putting Gore in the White House, he would live the rest of his life believing he paid back the Republicans for his loss by putting Hillary in the White House.