The plans for the rally first took shape over the summer. In July, one of the state’s most powerful conservatives, Lt. Gov. Dan Patrick, met with Mr. Trump’s political staff in Washington, trying to encourage the president to campaign in Texas to boost Republican turnout in November. Mr. Patrick was the Texas chairman of Mr. Trump’s campaign in 2016. Mr. Trump appeared to have listened, vowing on Twitter to hold a rally for Mr. Cruz in “the biggest stadium in Texas we can find.”

The rally was first scheduled for NRG Arena, a roughly 8,000-seat facility. After critics pointed out that there are high-school football stadiums in Texas that can seat far more people, the venue was changed to the larger Toyota Center, the home of the Houston Rockets basketball team, with a capacity of up to 19,000 for concerts. Republican officials said nearly 80,000 people had requested tickets.

In recent weeks, Republican leaders had openly questioned whether Mr. Cruz would win re-election. But on Monday at the rally, with thousands in attendance and Mr. Cruz leading Mr. O’Rourke in the polls, the Texas Republican bravado was back.

“Pundits say there’s going to be a blue wave, but I don’t think there’s even going to be a ripple,” said Sid Miller, the Texas agriculture commissioner and one of several Republican officials facing re-election who were at the rally. “I predicted a Trump win, a landslide, and was one of the few people that did, and I’m going to predict that we’re going to have another landslide in this midterm election.”

Throughout the afternoon, there were few signs of anti-Trump sentiment. Gwen Papillion, 43, works downtown and wanted to see what the crowd was like. She said she was an independent who voted for Mrs. Clinton in 2016 and planned on voting for Mr. O’Rourke for Senate.