Anthony Elps allegedly was holding a handgun as he locked eyes with Sonia Perez.
Perez, his ex-girlfriend, was holding her 4-month-old son in a car seat outside her West Philly home.
"Get in the car!" Elps said, according to police. He wouldn't ask again.
Elps snatched the car seat, baby and all, and tossed it in the back of his 1994 Pontiac Bonneville. Then he grabbed Perez by the neck and threw her in the car, too, said Highway Patrol Capt. Michael Cochrane.
When the Pontiac sped off, about 8:30 a.m. on Monday, Cochrane said, Elps turned to Perez and muttered, "I'm going to take you to Moo's house to kill you."
As they drove near 52nd Street and Lancaster Avenue, Perez, 25, saw a police cruiser parked at a Sunoco station.
Cochrane said Perez made the agonizing decision to jump out of the car - and leave her baby behind - and race towards the cop car for help.
Within seconds, a description of Elps and his car went across police radio.
Newly minted Highway Patrol officers James Pembroke and Brian Hilbert tracked Elps' Pontiac to 18th Street near Cayuga and captured him, Cochrane said.
The baby was found unharmed inside the car, but the cops didn't find Elps' gun. The identity of "Moo" is a mystery.
Elps, whose address was not immediately available, was charged with kidnapping, false imprisonment, simple assault, child endangerment and related offenses.
Cochrane praised Pembroke and Hilbert for tracking Elps down so quickly.
"Who knows what was in this guy's head? It was a reckless, reckless act," he said.
It's been a busy year for Elps, according to court records. He was arrested on Nov. 11 on charges that included robbery, simple assault and firearms violations.
On June 22, he was arrested on charges of aggravated and simple assault and carrying and instrument of crime. The charges were dismissed.