The massive pro-immigrant rallies around the country this spring conveyed a strong political message: "Today we march, tomorrow we vote." Will it come true? Many commentators think that it will--that the congressional debate over immigration has finally roused Hispanics to become more engaged in the political process. But it's far from certain that will happen.

Hispanics have never voted at the same rate as Anglos or African Americans. They cast just 6 percent of the ballots in the United States in 2004, barely half as many as blacks, even though they comprise a larger share of the national population. The standard explanation is that Hispanics simply lack the well-established political organizations needed to encourage registration and turnout.

There have been exceptions to the low-turnout pattern, notably in California in 1994, when a ballot measure aimed at blocking illegal immigrants from receiving most government services drove an unusually high Hispanic participation rate. At one point in the mid-1990s, a Democratic campaign consultant in California quipped that Latin American immigrants were turning out at a higher rate than Jewish professionals from Bel Air and Beverly Hills. But Latino participation subsided within a relatively short time.

A decade later, there are additional complications likely to affect Hispanic turnout in upcoming elections. Since 9/11, it's gotten a lot harder for immigrants to become citizens than it was during the late 1990s, when the Clinton administration did much to encourage the process. Even if illegal immigrants aren't termed felons, as would be the case under the bill that passed the U.S. House last year, they still won't have an easy time earning citizenship.

And citizenship is a key factor in the Hispanic-voting problem. Hispanic citizens don't, in fact, vote at a much lower rate than other groups. Once you subtract non-citizens, the percentage of voting-age Hispanics who went to the polls in 2004 was just under 50 percent-- less than for blacks and Anglos but by a relatively small margin. "People who look at the overall size of the Hispanic population and look at the vote think, 'Oh my God, what if these people ever get mobilized?'" says Ruy Teixeira, a Democratic polling expert. "But so many of these people can't vote anyway."

Then there's age. The median age of Hispanics in the U.S. is just 27, compared with 39 for Anglos. A much higher percentage of Hispanics don't vote because they're simply too young. That will change. About 750,000 Hispanics will turn 18 every year for the next 20 years. It could mean that Hispanic voting rolls will swell enormously--or it could mean that young Hispanics, just like young Americans of every race, will fail to exercise their right to vote in very great numbers.

Hispanics, in other words, already behave pretty much like everybody else. Given a sufficiently dramatic cause, they will turn out. But even if the immigration issue remains potent, age and failure to obtain citizenship are all but certain to depress the numbers for quite a while. And if Congress just punts and doesn't pass an immigration bill at all, it's even more likely that this spring's show of political strength will remain a seasonal fancy.