When it comes to illegal immigration, even some liberals understand the danger.

Best-selling novelist Richard North Patterson is the very photograph of modern liberalism. “He lives in San Francisco and on Martha’s Vineyard,” the book jacket says (now that’s a commute), “with his partner, Dr. Nancy Clair.” He’s also a former trial lawyer and chairs the liberal pressure group Common Cause. So when he speaks, we hear the voice of the modern American left.

In his latest novel, “Exile,” Patterson weaves a tale of a Palestinian terrorist bombing on American soil. How does the author get the terrorists into the U.S. undetected? Simple. They jet to Mexico, relax for a few days on the beach, then walk across the border into Texas. “Crossing would be no problem,” Patterson writes. After all, “thousands did it every day.”

But Patterson’s book, intentionally or not, also shows that border enforcement works. The terrorists considered crossing in western Mexico. But “self-appointed American vigilantes had begun spending their idle hours patrolling the borders of Arizona and New Mexico,” Patterson writes. The terrorists didn’t want to be “caught by some white people’s hunt for brown invaders they could not tell from Arabs.”

And with those words from a liberal novelist in mind, let’s turn to the Senate’s immigration “reform” bill.

Last week, the bill seemed dead when senators couldn’t agree to end debate and actually vote on it. Luckily for voters, the Senate had other important business to move on to: debating whether to vote that it had “no confidence” in Attorney General Alberto Gonzalez (although in the end, lawmakers couldn’t manage to vote on that, either).

But the immigration bill’s not dead. “I believe we can get it done,” President Bush announced. “I’ll see you at the bill signing.” Let’s hope not.

Senators who are pressing for a reform bill are making a fundamental mistake, one that’s highlighted by Patterson’s fiction. We don’t need a new immigration law. We need to start enforcing the immigration laws we have.

For starters, let’s complete the border fence.

Just last year Bush signed the Secure Fences Act of 2006. We need to finish building that fence, make sure it’s patrolled and give it time to work. And our brief experience with fences shows that they do work. Last year an observer wrote to tell me that his group (probably the “vigilantes” Patterson wrote disparagingly about) had “shut down approximately five miles (2 percent) of the California border. It obviously can be done.” They did it by building fences and then, along with federal agents, patrolling the border.

It’s long past time to close off the border. After all, the Pew Hispanic Center estimates there are 12 million illegal immigrants in the U.S. and that 40 percent (4.4 million) have been here less than five years. Thus they’ve arrived since Sept. 11. We have no idea who these people are, of course, or where they’re living.

What we do know is that there are a virtually limitless number of potential terrorist targets in the U.S. and that it’s impossible to protect all of them. At any time, a suicide bomber can detonate on a commuter train or in the middle of a busy store.

We’ve been protected so far by good police work -- authorities have broken up dozens of terrorist plots -- and by the fact that it’s difficult for our terrorist enemies to convince people to kill themselves. Simply put, self-detonation isn’t a growth industry.

But let’s suppose that a mere one-half of 1 percent of the illegals who’ve come here in recent years have terrorist leanings. That’s still 22,000 people. Considering it took only 20 terrorists to kill 3,000 Americans, that’s a scary number.

It wouldn’t take much to bring our economy to a halt. Suppose a handful of terrorists detonated at malls in Florida, Maine, Oregon and Texas. That would probably be enough to keep people out of shopping centers for months. Or if the same attackers blew themselves up commuter trains in San Francisco, D.C., Atlanta and Boston. Commuters would stay off public transit for weeks. We need to seal the border so we can keep the number of illegals from growing. That’s the first step. The next step is to shrink the number who are already here. That process will begin when we crack down on those who employ illegals.

The fact is that most illegals are coming here to work. Take away the jobs and they’ll return home. Again, lawmakers have already passed the necessary laws. They simply need to make sure those laws, including the REAL ID Act, are fully enforced. Let’s make it easier for employers to verify the immigration status of workers, and let’s hold those employers accountable when they don’t.

Many senators seem to think they need to pass another law before we can do anything about illegal immigration. But that’s not correct. We need to enforce the laws we already have. Only when we’re doing that should we even consider passing new laws.