WASHINGTON – It’s time to build a real fence or a wall along every foot of the 1,989 miles of the U.S.-Mexican border. There can be only two arguments against this approach to keeping out illegal aliens: (1) it won’t work – possible, but we won’t know unless we try; or (2) we don’t want it to work – then, we should say so and open our borders to anyone but criminals and terrorists. Either way, we need more candor in our immigration debates. Now is the time, because Congress is considering its first major immigration legislation in years.

In 2005, the Border Patrol stopped 1.19 million people trying to enter the United States illegally; 98.5 percent of them were caught along the Southern border. Of those who got through and stayed (crude estimate: about 500,000 annually), about two thirds lack a high school education. Even a country as accepting of newcomers as the United States cannot effortlessly absorb infinite numbers of poor and unskilled workers. Legal immigration already totals 750,000 to 1 million annually, many of them also unskilled.

I do not like advocating a fence. It looks and feels bad. It’s easily stigmatized as racist. It would antagonize Mexico. The imagery is appalling, but it beats the alternative: a growing underclass and social tensions. Moreover, a genuine fence would probably work. The construction of about 10 miles of steel and concrete barriers up to 15 feet high in San Diego has reduced illegal crossings in that sector by about 95 percent since 1992, reports Rep. Duncan Hunter, R-Calif., a supporter of a U.S.-Mexico fence. Sure, there will be tunnels and ladders. But getting in will be harder. Policing will be easier.

We also need to stiffen employer fines for hiring illegal aliens. Businesses should have to check prospective workers against computer databases with Social Security numbers, passports or immigration documents. Now, employers only have to inspect physical documents, which are easily forged. Even these lax rules are widely flouted and poorly policed. There are an estimated 10 million to 12 million illegal aliens in the United States.

Fewer jobs and genuine border control ought to curb illegal immigration. Good. Naturally, there’s another point of view. It is that the United States needs more unskilled workers to fill jobs native-born Americans won’t take. One solution is to admit more unskilled workers legally. By this view, Hispanics are assimilating economically and culturally as fast as some groups in the past.

Perhaps. But common sense and available evidence suggest skepticism. If there are “shortages” of unskilled American workers, the obvious remedy is to raise their wages. A Texas roofing contractor testified to Congress that he couldn’t get enough roofers at $9 an hour. OK, increase it to $10 or $12. Higher wages will bring forth more workers. Perish the thought. Business groups, led by the U.S. Chamber of Commerce, clamor for more “guest workers.” That’s a euphemism for cheap labor. These business groups seem unperturbed by extravagant increases in CEO pay. But they’re horrified by anything that might raise the wages of maids, waitresses, laborers or gardeners.

As for assimilation, it’s true that millions of Hispanic families are moving into – and reshaping – the American mainstream. But average trends look less encouraging. Since 1990, about 90 percent of the increase in people living below the government’s poverty lines are Hispanic. That has to be mainly immigrants and their American-born children. The median net worth of Hispanic households is about 9 percent of that of non-Hispanic whites (net worth is what people own minus what they owe), reports the Pew Hispanic Center.

Assimilation takes time. The big difference between today’s Hispanic inflows and past immigration waves is that those stopped. History or restrictive laws intervened. There was time for newcomers to adapt. Left alone, there’s no obvious reason why the present Hispanic immigration should even pause. Today’s unskilled arrivals make it harder for yesterday’s to get ahead. The two compete.

There’s a paradox. To make immigration succeed, we need to curb some immigration. That’s why it’s vital to control our border. It also explains why it’s important not to “solve” that problem merely by legalizing these huge inflows.

If we control new inflows, we should legalize the illegal immigrants already here. Many have American-born children, who are U.S. citizens. It is not desirable or ethical to try to force most illegal immigrants to leave. Yes, they broke the law; but we were complicit by making the law so easy to break. Their present shadowy status deprives them of rights and exposes them to exploitation. We should want the “melting pot” to work – and fear that it might come to a boil.