There was a striking moment in the Senate Judiciary Committee's debate on the Gang of Eight's comprehensive immigration reform bill when Republican Jeff Sessions and Democrat Charles Schumer argued over the number of immigrants who would be allowed into the country under the new legislation.

Sessions cited reports suggesting the figure would be more than 20 million over the next decade in addition to the 11 million or so who are already in the United States illegally. Schumer took issue with that, although he wouldn't name a figure of his own.

Then Schumer declared the whole dispute beside the point. "It is not that, 'Oh, this bill is allowing many more people to come into this country than would have come,'" he said. "They are coming. They're either coming under law or not under law."

The Democratic leader of the Gang of Eight continued: "This argument that there are going to be 20 million new people in this country under this bill ignores the fact that there are going to be lots of millions ... in the country illegally if we don't have a bill."

What made the exchange notable was that many Democrats who oppose more stringent border security measures argue that after recent increases in spending and resources the U.S.-Mexico border is already pretty secure -- "as secure now as it has ever been," in the words of Homeland Security Secretary Janet Napolitano. They also suggest that the number of illegal crossers is at an all-time low and will likely never rise again to levels seen in the 1990s and 2000s.

What Schumer conceded, perhaps in an unguarded moment, is that the border remains quite porous, and the U.S. can expect "lots of millions" to cross illegally in coming years if nothing more is done. The disagreement on Capitol Hill, of course, is over what should be done, but Schumer's off-the-cuff analysis provides a lot of material for Republicans pressing for a guarantee of greater security measures before millions of illegal immigrants are given legal status.

That GOP position received an even bigger boost with a recent report from the border in The New York Times. The crux of the story is that the number of illegal crossings is rising, and in response to greater security measures in Arizona, the flow from Mexico has shifted east to the Rio Grande Valley in Texas.

Yes, the number of illegal crossers is down from a dozen years ago as the U.S. economy remains a less powerful magnet than it once was. "But after nearly a decade of steady declines, the count has started to rise again over the past year," the Times reported. "The Rio Grande Valley has displaced the Tucson enforcement zone as the hot spot, with makeshift rafts crossing the river in increasing numbers, high-speed car chases occurring along rural roads and a growing number of dead bodies turning up on ranchers' land, according to local officials."

"There is just so much happening at the same time -- it is overwhelming," a Brooks County, Texas, sheriff's deputy told the Times.

Border Patrol agents are outnumbered; extensive, passable stretches of the border are unwatched; whole groups of immigrants cross unseen. "People are just crossing without fear," said an alderman in La Joya.

It's happening in part because the American economy, hit so hard by the economic downturn, is finally improving, becoming a draw again for immigrants, especially those from Central America who travel through Mexico on their way to the Texas border. Also, crime remains a terrible problem in many immigrants' home countries. And word is spreading that the U.S. Congress is contemplating a measure to legalize millions of illegal border-crossers.

That is the backdrop for this week's Senate debate on border security in the Gang of Eight plan. Democrats are dead set against any proposal that would make permanent legal status and a path to citizenship contingent on measurable improvements in border security. On the other side, many Republicans believe those improvements will never happen unless the law says legalization won't be allowed without security first. The only question is whether Republicans will stick to their guns or give in to Democrats.

In the debate, supporters of the Gang of Eight bill will almost certainly pronounce the border more secure than it has ever been; such rhetoric is a staple of such debates. But the situation on the border remains troublesome, and if the American economy continues to improve, as everyone hopes it does, the problem could become worse.

Schumer is probably right. In coming years, "lots of millions" will seek to come to the U.S. illegally unless something is done.