Shortly before Ronald Reagan was sworn in as president, his older brother, Neil, confessed to a reporter that their grandfather, Michael, had been an illegal immigrant. He’d fled famine in Ireland and, without papers, crossed into America by way of Canada, Los Angeles Times reported. Michael Reagan was “probably one of the early wetbacks,” Neil Reagan was quoted as saying. The Times issued the story after Ronald Reagan was elected president but before he was sworn in. The Asian edition of the Wall Street Journal responded with a satirical column suggesting that the presidential election be held over again.

Proceeding to inaugurate Reagan would cause a problem, the column quipped: “To all the millions of refugees hoping, plotting or conniving to find a way into America, this inauguration would stand as a beacon of hope. It would stand as a signal that no matter how many of America’s immigration laws you broke to get into America, no matter how tough your lot once you got there, if you manage to find someone to marry and the two of you manage to have children, you might just be begetting the parent of a President and if you do, the country will probably look the other way.”

We find ourselves thinking of the Reagan precedent in respect of President Obama’s announcement that he is going to look the other way in respect of illegal immigrants brought into America as children, so long as they stay out of trouble. Now his critics are complaining that this is going to send a signal that will invite others to come illegally with their children in the hope that, in respect of the children at least, a future government will someday look the other way. We, for one, hope so on both counts.

The problem with President Obama’s new policy on immigration is that it fails to go far enough. We prefer a path to citizenship. But even more than that we prefer an immigration policy that is promulgated in the context of a pro-growth economic policy that will create the conditions in which it is apparent that we need more people. We need more people like, among others, those to whom Mr. Obama is preparing to give a pass. But Mr. Obama isn’t marking the point in the context of pro-growth economics.

Reagan never missed that chance. It came up in 1984 in the debate between Reagan and Vice President Mondale. That’s the debate in which Reagan declared: “I believe in the idea of amnesty for those who have put down roots and who have lived here even though sometime back they may have entered illegally.” Reagan was always pressing pro-growth points — lower taxation, particularly at the top margin, deregulation, free trade, a strengthening dollar. He understood the idea of human capital. He understood that a soaring population was good for a country.

At one point in the debate, the newspaper columnist Georgie Annie Geyer, asked Reagan: “Do you ever feel that we are in for an Armageddon or a situation, a time of anarchy, regarding the population explosion in the world.” Reagan was ready. “No. As a matter of fact, the population explosion, if you look at the actual figures, has been vastly exaggerated — over exaggerated. As a matter of fact, there are some pretty scientific and solid figures about how much space there still is in the world and how many more people we can have. It’s almost like going back to the Malthusian theory, when even then they were saying that everyone would starve with the limited population they had then.”

This is the worldview lacking in Mr. Obama. He’s not the only president whose efforts at immigration reform were frustrated by Congress. President George W. Bush had an even better, more liberal program than Mr. Obama offered. He was defeated by opponents on both the left and right. We’re all for a president, Mr. Obama included, asserting the maximum of his powers, though we’re no great fans of by-passing Congress to make up for a failure of leadership. Reagan won his victories the old fashioned way, by leading, both in the Congress and in the national debate.

This is an opportunity for Governor Romney. In the Republican primary debates, he fell short on immigration, allowing himself to get drawn in the question of his gardener. The other candidates were worse. The only one hewing a principled line was Congressman Ron Paul. The opportunity is to revive the argument for immigration couched in the comprehension that restrictions on immigration are protectionist and therefore anti-growth. One of the problems with Mexico might even be that immigration is falling off.

This was reported in April by the Christian Science Monitor, which quoted the founder of the Mexican Migration Project at Princeton University, Douglas Massey, as having documented what he calls “net zero” migration between America and Mexico. Mr. Massey, according to the Monitor, reckons that the population of undocumented immigrants in America fell to approximately 11 million from 12 million during the height of the financial crisis between 2008 and 2009. Since then, the Monitor reported, Mexicans without documents “aren’t migrating at rates to replace the loss, creating a net zero balance for the first time in 50 years.”

We aren’t indifferent to the violence and lawlessness at the border. We believe in the distinction between border security and immigration, a distinction marked so well the other day by the Republican nominee for Senate in New Mexico, Heather Wilson. She’d be a good person for Mr. Romney to talk with. In any event, net zero immigration with Mexico is a trap Mr. Romney will want to anticipate. If he gets elected and wins the tax, regulatory, and monetary reforms needed to pull us out of the doldrums, he won’t want to discover that we can’t get the human capital we so desperately need. Or bring in the grandfather of a future president.