TUCSON -- Michael Chertoff was in the driver's seat of a white Chevrolet Tahoe, under the glare of high-powered lights ringing Border Patrol headquarters. It was 10 p.m., 15 hours into the Homeland Security secretary's workday.

An agent sitting beside him tapped a glowing computer screen. A map expanded.

Drawing on an arsenal of radar, sensors and cameras, it displayed a spray of red dots -- suspected border crossers.

But Chertoff saw that the "virtual fence" had a major flaw: It wasn't able to show in real time where agents were on patrol along the border.

The secretary, leaning back wearily in his seat, said pensively, "We'll work on this."

In the three years since Chertoff took office, his job has been transformed by a bitter debate over illegal immigration that made sealing the border a priority. Once dubbed the nation's "anti-terrorism czar," he is now also its top border agent.

The vehicle-mounted computer is just one piece of Chertoff's efforts to revolutionize the nation's border with Mexico. Besides the installation of high-tech surveillance tools meant to create a virtual fence, he is spearheading the construction of 670 miles of real fence and a rapid expansion of the Border Patrol.

As he pursues the ambitious agenda, the secretary must convince skeptical Americans that it will work. Chertoff -- a graduate of Harvard Law School and a former Supreme Court clerk, federal judge and federal prosecutor -- retains a lawyer's faith in the power of a good argument persuasively delivered.

"I always believed that if I could get direct access to 12 people, I could talk sense into them," he said.

Chertoff thinks he has the grit to get the job done: "I'm really, really stubborn. That and, honestly, I guess we're very conscious of the fact that it's very easy to get bogged down."

This month, making use of the powers given to him by Congress, Chertoff announced that his department would bypass federal laws to speed construction of 370 miles of fence, angering environmentalists and border groups.

"To me, the most important thing we're doing at the border is showing the American people that if we make a judgment that we need to do something and we promise to do it, we'll do it," Chertoff said.

The visit to the Tucson station was Chertoff's third stop on a recent 48-hour, two-state blitz through border country, a trip that vividly illustrated the enormous task ahead as he races toward the end of his tenure.

Yuma, Ariz.

Luis Aguilar's family and colleagues waited for Chertoff in the Border Patrol's sector headquarters. The agent died in January when an alleged drug dealer fleeing to Mexico in a Hummer struck him.

Aguilar's 5-year-old daughter, knobby-kneed in tights and black patent-leather shoes, piped up as Chertoff entered. "Why is everyone clapping?" she asked. "For Daddy?"

The 32-year-old was the first agent to die violently in the line of duty since 1998. As the Border Patrol has put more agents on the front line, violence has increased. Agents were assaulted 987 times in fiscal year 2007 -- with Molotov cocktails, rocks, gunfire, fists and, as in Aguilar's case, vehicles -- a 31% increase from 2006.

"An unfortunate metric," noted Chertoff, who said it indicated smugglers were feeling the heat of more enforcement.

Chertoff wants 18,000 border agents by the end of the year, up from about 15,300, double the number when President Bush took office in 2001. To get those boots on the ground quickly, training has been compressed from five months to at most 95 days.

Border officials are concerned about their ability to offer enough field training. Critics say the accelerated training adds to the danger by leaving agents unprepared.