Texas Gov. Rick Perry and Lt. Gov. David Dewhurst probably wouldn't admit it, but they want to engage in social engineering.

They want people not to waste money on drugs, liquor, smokes or lottery tickets.

Not everybody, mind you, but poor people.

That's because poor people can get taxpayer dollars to help feed their kids or tide their families over when they lose a job -- and we can't have them spending that money on things the rest of us disapprove of.

I can understand the sentiment: The public wants its investments to help people, not enable their bad -- or illegal -- habits. And I'm a fan of personal responsibility, too.

But the method of choice put forward by Perry, Dewhurst, Sen. Jane Nelson of Flower Mound and other Republicans is to drug-test people applying for public benefits such as Temporary Assistance for Needy Families and unemployment assistance. As though drug use is the main reason people with kids are poor or have been thrown out of work.

It would be naive to deny that some of Texas' neediest children live in dysfunctional households with substance-abusing adults. And, sure, some Texans can't keep jobs because of their addictions.

But setting drug testing as a hurdle to qualify for public assistance runs into an inconvenient reality called the Fourth Amendment, which bars the government from conducting unreasonable searches.

The Supreme Court has ruled a number of times on what an "unreasonable" search looks like. So far, it doesn't look like a drug test to control how people on welfare spend that money.

In 1989, the justices ruled that the government can reasonably require railroad crews to undergo random drug tests because of a "special need" having to do with public safety.

In 1995, the court said public schools can test student athletes for illegal drugs. That was extended to nonathletes in extracurricular activities in 2002. Students do lose some rights at the schoolhouse gate.

But in 1997, the court said Georgia couldn't require political candidates to take a urine test to get on the ballot. Voting 8-1, the court said the state invaded personal privacy "for a symbol's sake" when law enforcement could adequately deal with any real problems involving office-seekers.

Where does what Texas is considering fall?

A federal appeals court ruling in 2003 resulted in Michigan's welfare drug testing law being voided. And in October, a U.S. District Court judge blocked Florida from enforcing its 2011 law while a challenge filed by a single father who's a military veteran and student works its way to a trial now set for March.

This week, Nelson pre-filed SB 11, which would require TANF applicants to undergo a "controlled substance use screening assessment" to determine whether there's "good cause" to send them for a drug test.

That presumably is to get over the legal hurdle of requiring "particularized suspicion" to invade someone's privacy with a search.

Before the Legislature jumps all in on this, there are difficult questions to answer.

Perry says the state will save money by paying out less. Well, yes, if people who don't want to be tested don't apply, that's fewer dollars spent -- but how does that help the kids who still need assistance?

And how does it make sense to use TANF funds, which the federal government sends to the states, to conduct drug tests (as Nelson's bill directs) instead of to help poor children?

There's considerable debate about how many drug users will get caught by testing or how many will be deterred. Federal law already allows states to ban those with felony drug convictions from receiving welfare, and Texas does. The Fourth Amendment doesn't prevent prosecution of welfare recipients for drug crimes.

Perry said Texas "will have an efficient program." But will it be a constitutional one?