Those who saw mass migration from Mexico as a threat and those who did not all agreed on one thing: It was unstoppable without dramatic action by the federal authorities. They turned out to be wrong about that. The title of a new report from the Pew Hispanic Center, “Net Migration From Mexico Falls to Zero — and Perhaps Less,” says it all.

So everyone buckle your seatbelts, if indeed more Mexicans are going back to Mexico than crossing into the United States. It would change a whole bunch of calculations in presidential campaigning. And it would make whatever the Supreme Court says about Arizona’s tough immigration law less consequential.

The reason for this changed pattern matters most, and activists on the issue have an interest in pushing their own explanations. “The anti-immigrant climate” may account for some of it, Jennifer Lee of Colorado Legal Services told The Denver Post. Ignoring, as many advocates do, the difference between legal and illegal, she fools nobody.

Immigration-control groups link the trend to a weak economy and lack of jobs. That makes it temporary and thus keeps them in business. To see the changes as permanent, argues Mark Krikorian, executive director at The Center for Immigration Studies, is “wishful thinking by people who just want amnesty.”

Which, actually, very few Americans want. While there are those on the far left and the cheap-labor right who “just want amnesty,” they are a minority according to every reputable poll. Most who want to put illegal immigrants “on the path to citizenship” also insist that this amnesty be the last. That means it must be paired with serious workplace enforcement.

In any case, this trend is not temporary.

Demographers following the plunging birthrates in Mexico have been predicting this day would come for some time.

There’s now a sharp reduction in the number of 18- to 35-year-old Mexicans — the age group most likely to come here illegally.

“Their number was a huge bulge and is receding,” Roberta Jacobson, assistant secretary in the Bureau of Western Hemisphere Affairs, told the Association of Opinion Journalists at a State Department briefing on Monday. While rising unemployment is the immediate trigger for the sharp decline in immigration from Mexico, she said, the drop-off in illegal entrants “will probably continue.”

The more manageable numbers can only help advance the sort of comprehensive immigration reform that got wedged into America’s left-right divide. The lower boil could help Republicans seeking Latino votes in swing states moderate much of their party’s no-pity position. Likely presidential nominee Mitt Romney is already softening some harsh views expressed when he needed to appease the party’s base.

Another factor in stopping the wave of illegal immigration was beefed-up enforcement, the Pew report noted. President Obama was the first president in decades to start seriously going after employers hiring undocumented workers. Rougher state laws undoubtedly played a part, but some are rather ugly. How preferable that Americans trust the federal government to enforce the immigration laws, which is its job, after all.

Could America be close to actually solving one of its vexing problems? Smart reform of our immigration laws would do the following: It would protect our native and legal immigrant workers from unfair competition. It would let us devise an immigration program that meets our need for more skilled workers. And it would restore some peace at the border.

Someday, Mexicans and Americans may be able to easily cross into each other’s countries for business, visiting, shopping or dinner. If the pressures at the southern border are starting to ease for sure, then that day may come sooner than we thought.