James made a 3-pointer with 1 minute left in overtime and blocked a 3-point attempt by Texas A&M's B.J. Holmes in the final seconds that would have tied the game.
By the time he was finished, James was exhausted but happy to know that Texas would still be sitting at No. 1.
"I'm so tired," James said. "They came out and punched us in the mouth and we didn't retaliate until the second half."
Texas (17-0, 3-0 Big 12) had earned its first No. 1 ranking with a dominating start in which few teams could even stay close to the Longhorns. Then they had to fight through a tough road win at Iowa State before wrestling with their big rival on their home court.
"In the position we're in right now, the other team's emotions are going to be harder no matter where you're playing," Texas coach Rick Barnes. "It was a hard-fought game, but I don't know what anybody else expected."
The Longhorns didn't grab the lead Saturday night until James' first basket of overtime. A 13-point deficit in the first half was the Longhorns' largest of the season.
"We fought back," James said. "Once we put it in overtime, we felt like it was our game."
Donald Sloan scored 21 points and Bryan Davis added 17 for the Aggies (12-5, 1-2) who dropped their second straight in Big 12 play after nearly earning the program's first win over a No. 1 opponent.
"We didn't come here for a moral victory. We came to win," Sloan said.
The Aggies had the game plan to do it. Texas A&M shut down hotshot Texas freshman Avery Bradley and senior forward Dexter Pittman and held the Longhorns to 35 percent shooting.
Johnson kept Texas in it early, scoring nine of his 18 points in the first half.
James scored 23 points in the second half and overtime. The Big 12's career rebounding leader also grabbed a game-high 12 boards. "He did what seniors do," Texas A&M coach Mark Turgeon said. "They have so many players, so many weapons."
Texas A&M was in control and had taken the home crowd out of it when Davis spun around Pittman for a layup that gave the Aggies a 10-point lead early in the second half.
But Davis, who would eventually foul out in overtime, soon picked up his third foul and went to the bench. James pulled Texas to 44-37 with a 3-pointer and followed it with a three-point play after a steal.
Another jump shot by James and his soaring dunk pulled Texas to 48-46. He scored 12 points during a 15-6 Texas run.
J'Covan Brown's jumper for Texas tied it at 60 and the Longhorns had a chance to win in the final seconds of regulation.
Texas A&M tried to set up for the winning shot but Sloan was stripped of the ball. Bradley sprinted to the basket but his layup skirted around the rim and Texas' Alexis Wangmene and James missed tip-in chances to win at the buzzer.
In overtime, James and Sloan traded baskets and Texas A&M led 67-66 on David Loubeau's dunk with 1:18 left. After James made his 3-pointer, Dogus Balbay made one of two free throws. His only point of the game put Texas up 70-67 with 30 seconds left.
On their final possession, the Aggies swung the ball to Holmes in the left corner but James leaped and stretched out his hand to block the shot. Brown grabbed the ball, got fouled, and made two free throws with 3 seconds left for the final points.
"I was like 'He ain't getting the shot off," James said. "It was a big play."
Big enough that Texas gets to stay at No. 1 for at least a few more days. The Longhorns play at No. 13 Kansas State on Monday before traveling to No. 15 Connecticut next weekend.
"This is what you play for," James said. "This is how you make a name for yourself as a team."
