It’s the walk of danger.

I park my car near the United States Capitol nearly every morning and hustle up to one of its five main entrances. As I stride toward the building, I’m often on the phone or dialing through my BlackBerry, clearing off a host of emails that spilled in during the commute. But other times, I’m not as distracted and I’m looking around. And I stare at the Capitol Dome pitched against the sky and think, what if….

The “what if” part is what’s scary.

By its nature, the United States Capitol must be an open place. Openness to democracy and America’s way of government is the Capitol’s greatest strength.

It’s also its greatest vulnerability.

And that’s where the “what if” comes in.

Sure, the Capitol complex is fortified with barricades and a phalanx of machine-gun wielding cops. The inside is secure. But outside, pretty much anything could happen.

I’m usually pre-occupied with “what if” when my BlackBerry is dormant during those morning strolls. That’s when a stream of scenarios starts circulating through my mind. These are threats which could easily disrupt my morning ritual. And the rituals of thousands of others who work on Capitol Hill.

I consider the possibility of a mad man spraying the building with a hale of bullets, picking off hundreds who are just walking to work. I think about someone lobbing a Molotov cocktail toward the Senate wing of the Capitol. And then, there are suicide bombers.

On Friday, police say that 29-year-old Moroccan native Amine El Khalifi aimed to execute precisely the final sequence which pre-occupies me during those morning walks. Khalifi thought he was outfitted with a suicide pack and would stride up to the Capitol and sacrifice himself, perhaps killing “30 people” in the process.

Khalifi’s carnage would be indiscriminate. It might take out a few U.S. Capitol Police officers. A junior staffer who handles a freshmen Congressman’s schedule. A longtime aide who’s versed in the minutiae of poultry policy for a subcommittee. A Capitol maintenance guy. A family from New Mexico visiting Washington for the first time. A journalist who writes about aviation. A 19-year-old intern who’s just caught Potomac fever. A jogger who lives near Eastern Market. Maybe a lawmaker or two.

These are the people who come and go daily from United States Capitol. And if Khalifi had his way, some of them would be dead.

But the victims aren’t the only targets.

The United States Capitol is a global icon. Effecting a suicide bomb on the grounds of the Capitol is about as brazen and brutal as it gets. Granted, the FBI and Joint Terrorism Task Force were completely on to Khalifi, even outfitting him with fake explosives. The public was never in any danger. But Khalifi’s intent was clear. Kill and reap mayhem at the doorstep of the most-visible symbol of democracy on the planet.

Most House and Senate lawmakers beat a hasty retreat out of Washington early Friday afternoon once both bodies passed the payroll tax break measure. Many Members of Congress weren’t even aware the FBI and U.S. Capitol Police pulled off the Khalifi sting. But those who lingered were clear about what Friday’s events meant.

Appearing on FOX, House Homeland Security Committee Chairman Pete King (R-NY) indicated that security officials generally regard a suicide attack on U.S. soil as a remote possibility. But that may have changed Friday.

“I think a line has been crossed,” said King. “I think we’re going to have to re-think some of our conceptions.”

Rep. Sheila Jackson Lee (D-TX) serves on the Homeland Security panel alongside King. She noted that the U.S. periodically experiences reminders of the post 9-11 world. She cited the effort to bomb Times Square a few years ago.

“I wouldn’t say it’s a wakeup call. But I would say it’s an ongoing affirmation: we live under threat,” said Jackson Lee. “Jeopardy is not far away.”

Attempted terror attacks on U.S. soil have been limited since 9-11. Certainly there was the Times Square effort, the Christmas Day bomber and the shooting at Fort Hood. But Rep. Emanuel Cleaver (D-MO), who chairs the Congressional Black Caucus (CBC) said these incidents may be disparate. But the motive is the same.

“There are people who would like to destroy us. And not because we are Democrats or Republicans or conservatives or liberals or tea party or CBC. They want to destroy Americans. And today is just another stark reminder that they are out there,” Cleaver said. “We have people everyday who go around and preach hate. And when you mix hate with religion, its impact is unlimited.”

While some described the Khalifi case as a new platform in the war on terror, others believed it reflected something more positive. For years after September 11th, security and intelligence experts criticized the U.S. government for being reactive instead of pro-active when it came to threats. Such was the case when airports began to require people to take off their shoes following the shoe bomb attack.

At least one Member of Congress hailed law enforcement for getting to Khalifi early.

“This is really wonderful intelligence. He didn’t have anything on him. His explosives were phony,” exclaimed Washington, DC’s non-voting representative to Congress, Del. Eleanor Holmes Norton (D-DC). “Tell me there’s no better intelligence than that.”

The intelligence which bore into Khalifi’s intent impressed many. But Emanuel Cleaver said that Friday’s episode revealed another pivot.

“I told my staff this is a reminder to me that maybe when we’re walking around here on the (Capitol) campus, even with all the security, we all need to be a little more careful,” Cleaver said. “I’m going to be a little more observant about what’s going on around me.”

The temperature peaked in Washington at an unseasonably warm 54 degrees Friday. So many lawmakers and aides strolled outside when they trekked between the House and Senate office buildings and the Capitol. But Cleaver believed the threat outdoors could deter that practice After all, an anti-health care reform demonstrator spat on the Missouri Democrat in March, 2010 as he walked outside the Cannon House Office Building. Cleaver said he might resort to taking the coil of underground tunnels which snake between the Capitol and the office buildings.

“Maybe we’re going to have to walk around with a little higher level of paranoia,” Cleaver said.

Norton dismissed Cleaver’s suggestion.

“I would not on a good day fear to go out and walk across to the Capitol,” declared Norton. “Not after what I saw here.”

And that’s the internal battle waged by those who work on Capitol Hill.

The U.S. Capitol will always be one of the highest-value targets in the world. Yet it’s also one of the most secure. But because the Capitol falls in the crosshairs, the area outside carries an inherent risk. People interpret that risk differently as exhibited in the remarks of Cleaver and Norton.

Every job carries risks. Lots of those who work on Capitol Hill never give security a second thought. Others are very aware. They know where the emergency escape hoods are located. Some keep emergency food and water in the office. Some female aides stash a pair of flats under their desk in case they need to evacuate. Another staffer once told me she never wears a pair of heels to work that she can’t run in.

Certainly there are other terrorist targets: the Golden Gate Bridge, the Empire State Building and countless sports arenas. High-profile restaurants where lawmakers and diplomats dine in Washington now appear to have crept onto the radar screen.

But Friday’s bomb plot unveiled that the most-dangerous aspect of the job might not be just the fact that one works on Capitol Hill. It may be the walk to and from there instead.