Byron York

A fight over the future of Social Security has emerged as a key feature of the struggle between Republican presidential candidates Rick Perry and Mitt Romney. But there is another issue that divides the two and has not received enough attention: immigration.

In the Sept. 7 Republican debate, Perry, governor of a border state for more than a decade, didn't have much to say about immigration. When asked how he would secure the U.S.-Mexican border, his answer was "boots on the ground" -- Border Patrol agents -- and more unmanned drone aircraft. That was about all Perry had to say, beyond criticizing President Obama. "For the president of the United States to go to El Paso, Texas, and say that the border is safer than it's ever been, either he has some of the poorest intel of a president in the history of this country, or he was an abject liar to the American people," Perry said. "It is not safe on that border."

But what to do about it? Romney was more prepared for the question. First, he called for a fence along the border. Then, he called for more U.S. agents to secure the fence. And then he called for measures to turn off what he called the "magnet" drawing illegal immigrants to the United States. "Sanctuary cities, giving tuition breaks to the kids of illegal aliens, employers that knowingly hire people who are here illegally -- those things also have to be stopped," Romney said.

Romney seemed clearly ready for a confrontation with Perry over immigration, but the debate moved on, and the moment passed. But there will be a reckoning, and probably soon, because on a number of key immigration issues, Perry's positions will alienate a significant part of the Republican primary electorate. Romney or some other candidate is sure to take advantage of that.

Start with the border fence. Perry opposes it. "Building a wall on the entire border is a preposterous idea," he said recently in New Hampshire. "The only thing a wall would possibly accomplish is to help the ladder business." Perry says he supports some forms of "strategic fencing in certain urban areas," but that's all.

Then there are measures to stop employers from hiring illegals. Perry opposes E-Verify, which is a program requiring employers to check the legal status of new hires. It has been very effective in stopping the hiring of illegals, but Perry does not support requiring private businesses to use it, and he doesn't want state agencies in Texas to use it, either. "E-Verify would not make a hill of beans' difference in what's happening today," Perry said in a 2010 debate.

Byron York

Byron York, chief political correspondent for The Washington Examiner