During the 2016 campaign when both were competing for the Republican presidential nomination, the race grew bitter at times, with Trump dubbing Cruz "Lyin' Ted" and Cruz calling Trump a "snivelling coward."

Monday's rally in Houston was the first time Trump threw himself at helping Cruz fend off O'Rourke, even though Cruz has opened a lead in opinion polls on O'Rourke.

"God bless Texas, and God bless President Donald Trump," Cruz said in opening remarks on Monday.

Having given Cruz a new nickname, "Beautiful Ted," Trump reviewed their 2016 battle when he took the podium after the two men embraced.

"You know we had our little difficulties," he said. "I tell you what, nobody has helped me more ... than Senator Ted Cruz."

Trump quickly turned his attention to O'Rourke, who is running strong in a Republican state and giving Democrats hope they can break their opponents' longtime stranglehold on Texas.

Trump called O'Rourke "a stone-cold phoney" and a "radical, open borders left winger."

Trump is campaigning to stave off a Democratic push to take control of the US House of Representatives and possibly the Senate, when they could derail or stall much of his agenda and increase congressional oversight and investigation of his administration.

Democrats have seemed poised for months to capture the House, but many races have tightened in recent weeks to the point that some analysts think it is conceivable Republicans could hang on to control.

"About a month ago, they were talking about this 'blue wave.'" Trump said of Democrats' anticipated gains. "We're not hearing that anymore. The blue wave is being dissipated a little."

Buoyed by a recent uptick in his job approval ratings to the high 40s in opinion polls, Trump is hammering away at two major themes: illegal immigration and the brutal Senate confirmation battle over US Supreme Court Justice Brett Kavanaugh. He is also promoting a plan for middle-class tax cuts.

Australian Associated Press