You've heard this before. A political earthquake is coming--and soon. Young people in their late teens and early 20s are ready to vote like never before. They're a new and different generation, inclined to vote Democratic in large enough numbers to precipitate a political realignment that could make Democrats the majority party for years and years to come.

Indeed, this could happen. But Republicans shouldn't panic yet. Political projections like this have a history. In 1972, with 18-to-21-year-olds permitted to vote for president for the first time, George McGovern thought a tidal wave of young people would elect him president over Richard Nixon. McGovern lost by 23.2 percentage points. In 1992 Arthur M. Schlesinger Jr. declared that Bill Clinton's election had touched off a new era of Democratic rule. Two years later, Republicans won both houses of Congress and numerous governorships and state legislatures in a historic landslide.

So there's reason enough to be skeptical of a realignment led by young folks (or anyone else), but not dismissive. The millennial generation, consisting of those born since 1982, is the largest such cohort in history, bigger than the Baby Boomers (born from 1946 to 1964) and not as conservative as Generation X (1964 to 1982). And millennials, in their initial surges to the polls, voted slightly Democratic in 2004 and overwhelmingly (60 percent) in 2006.

In Millennial Makeover, Morley Winograd and Michael Hais argue that this is just the beginning.

America is about to experience another electoral upheaval, or realignment, just as it has throughout its history. .  .  . If Democrats can maintain this initial generational allegiance during the next two presidential elections, they should gain a decisive electoral edge for decades to come.

Perhaps they will, but Winograd and Hais aren't -unbiased observers. Both are California Democrats with extensive political experience, and they indulge at times in Democratic spin. But they have a case, based partly on current political circumstances in which Republicans are retreating and even more on the nature of the new generation: "What does seem clear," they write, "is that the Democrats' approach to political and social issues appears more compatible with Millennial attitudes."

At least for now, I'd add.

How so? Winograd and Hais adopt the generational theory of history invented by William Strauss and Neil Howe that is too complicated to go into here. Suffice it to say, the millennials (aka Generation Y) fit the theory perfectly as civic-minded young people who are socially tolerant, optimistic, academically accomplished, supportive of activist government, and surprisingly partisan. Their time has come and, as luck would have it, right in line with the Strauss-Howe theory.

What's particularly important is their technological savvy. "The political world is about to be shaken to its core by the arrival of these new capabilities for reaching voters, especially the generation that uses them every moment of every day," Winograd and Hais insist. We're talking here about the Internet, YouTube, and iPods, plus the online social networks MySpace and Facebook. For millennials, these are the preferred channels for news and information and for communicating with friends.

Democrats are far ahead of Republicans in using these tools, both to connect with voters and raise money. This has ominous implications. Republicans once led in fundraising through direct mail to millions of small donors and in winning elections with massive TV advertising. But these are less effective today. In 1965 a party could reach 80 percent of 18-to-49-year-olds with three 60-second, prime-time television commercials. Now it would take 117 of these TV spots to achieve 80 percent coverage. The Internet is not only an easier way to contact young people, it's cheaper.

"History suggests," according to Winograd and Hais, "that those who find ways to integrate the new technology with existing tactics to produce multi-faceted campaigns that reach all voters will be especially successful in future elections." Barack Obama's presidential campaign is the reigning example.

All of this sounds fine until Winograd and Hais get to the shaky foundation on which their realignment scenario rests: "Once individuals take on a party identification," they write, "they don't often change it and, as a result, a rising new generation spearheads major shifts in party identification and the political realignment that flows from it." But the truth is that Democrats don't yet own the millennial generation, and may never.