Cruz, for his part, has called Trump a "snivelling coward" and a "pathological liar."
After declining to endorse Trump during the Republican National Convention, he changed course in September 2016 and begrudgingly offered his support.
Today, bygones will be bygones as Trump and Cruz appeared together onstage at the Toyota Centre in Houston.
Trump acknowledged that he and Cruz had a "very, very nasty and tough campaign," but he said they have "worked together very closely" since then.
"I like him a lot. I actually like him a lot, and he's a very smart guy. He loves the people of Texas," Trump said.
The President also said he believes O'Rourke is "highly overrated."
The Democrat drew a crowd of 55,000 people at a rally in Austin with country music star Willie Nelson last month and pulled in a staggering US$38.1 million during the third fundraising quarter, more than three times Cruz's total of US$12 million during the same period.
"What I heard about him, I figured he must be something a little special. He's not," Trump said, adding that O'Rourke "got beaten badly" in his debates against Cruz.
Trump headed to Cruz's home city to boost momentum for his onetime foe on the first day of early voting in Texas.
The rally was moved to the Toyota Centre in downtown Houston after organisers said ticket requests swamped the seating capacity at NRG Arena, the initial venue. A Trump campaign spokesman said the smaller NRG Arena, which accommodates fewer than 10,000 people, was the only facility they could secure in time.
The Toyota Centre, home of the Houston Rockets, seats more than 18,000 people.
