Americans are just growing accustomed to the impact of the massive millennial generation as consumers, workers and voters. But now the crusade for gun control led by survivors of the Parkland, Florida, school shooting is heralding the arrival of the younger generation rising behind them. And in their racial diversity, attitudes toward religion and culture, their digital fluency and their political priorities, this emerging generation -- usually called the post-millennials, sometimes labeled Generation Z -- might shake American life even more profoundly than the millennials.

"The millennial generation is just the cutting edge, the front line of what this country is going to be becoming," says William Frey, a demographer at the Brookings Institution who has extensively studied the millennial and post-millennial generations. "The millennials are the tip of the iceberg, but rushing through for the next several decades with the post-millennials is going to be much more diversity and a very different America than we've known in the past. I think this is a big sea change for our country."

For Republicans who have nervously watched polls showing an enormous backlash against President Donald Trump's insular nationalism among the millennials, this younger generation could represent an even greater threat. By 2024 -- just two presidential elections from now -- the generation of young people that includes the students organizing a march on Washington next month to demand gun control will represent 1 in every 10 eligible voters.

"It's difficult to see how the GOP will gain ground among younger adults as this newest cohort continues its demographic and cultural drift away from the average GOP voter," says Daniel Cox, research director of the nonpartisan Public Religion Research Institute, which recently conducted one of the most extensive surveys of attitudes among post-millennials in partnership with MTV. "The cultural disconnect is only going to grow. The average GOP voter is a middle-aged white Christian. That's simply not what the US is going to look like in the future."

As with many generations, there's no universally agreed-upon definition of when the millennials end and the post-millennials begin. Most definitions of the millennial generation start with young people born in 1981. Some demographers, such as those at the Pew Research Center, say the millennial generation stops and the post-millennials begin with anyone born in 1998 or after; another, somewhat more widely accepted, definition starts the post-millennial generation with young people born in 2001 or after.

Using the first definition, of people born in 1998 or later, the post-millennials (at 77.6 million people) are already slightly bigger than the millenials (at 75.5 million people), according to Frey's calculations. Placing the generational dividing line at 2001 shifts the balance toward the millennials, with 88 million people, as compared with 65 million post-millennials. By the first definition, the post-millennials are already the nation's largest generation; by the second they are almost identical in size to Generation X, and smaller than the millennials and the baby boomers.

Either way they are a big cohort, and are poised to rapidly extend their impact on American society -- and politics -- over the next decade.

The campaign for greater restrictions on gun access led by students who survived the mass shooting at Marjory Stoneman Douglas High School has highlighted one attribute of the post-millennials: their digital fluency. Even compared with the millennials, this younger generation has marinated in a world of ubiquitous communication and pervasive social media: Using the earlier cutoff date, the oldest post-millennials were younger than 10 when Steve Jobs introduced the first iPhone in January 2007. In the shooting's aftermath, the students have shown themselves to be skilled and utterly fearless in leveraging social media to build their movement and advance their arguments.

"We should change the names of AR-15s to 'Marco Rubio,' " one student tweeted after the senator from Florida rejected their calls to support a ban on assault weapons, "because they are so easy to buy."

We should change the names of AR-15s to "Marco Rubio" because they are so easy to buy. — Sarah Chadwick// #NEVERAGAIN (@sarahchad_) February 23, 2018

Another defining attribute of the post-millennial generation is its kaleidoscopic racial diversity. It has instantly displaced the millennial generation as the most diverse in American history. Frey has analyzed the racial composition of the post-millennial generation using the starting line of 1998. By that definition he calculates whites make up only 51.5% of the post-millennials, with kids of color representing fully 48.5%. That racially diverse population is led by Hispanics (about 25% of the entire generation) and African-Americans (about 15%), but also includes substantial numbers of Asian-Americans and mixed-race young people (about 5% each).

This represents an increase in diversity compared with even the millennial generation (which is just under 45% nonwhite by Frey's calculation), much less earlier generations. Nonwhites represent only about one-third of the population in all generations older than the millennials.

Because the post-millennials will continue to evolve, primarily with new births but also with some immigration, Frey projects that by around 2025 nonwhites will constitute a slight majority of the generation. If that occurs, it will mark the first time in American history that whites represent a minority in any generation. That prospect is why Frey calls even the highly diverse millennials a "bridge" to the nation's increasingly diverse future . Already, by Frey's calculations, a majority of post-millennials are kids of color in 13 states and 38 of the 100 largest metropolitan areas, ranging from Los Angeles, San Antonio and Houston to New York, Chicago and Memphis.

Because so many of the post-millennials are native-born, rather than immigrants who must wait to obtain citizenship, they will steadily become eligible to vote as they turn 18 in the years ahead. The nonpartisan States of Change project has produced detailed forecasts for the future composition of the eligible voter population. One of its most striking projections is that in 2018, millennials will become the largest generation of eligible voters, displacing the baby boomers, which have held that premier position since 1978.

Millennials will remain the largest cohort of eligible voters for some time, the project forecasts. But the post-millennials will steadily join them through the 2020s as a measurable force. Using the generational definition that begins in 2001, the project forecasts that the post-millennials will represent about 3% of eligible voters in 2020, 10% in 2024 and 16% in 2028. By 2028, the millennials and post-millennials combined will equal just about half of eligible voters. Because older people vote at higher rates than younger ones, the two generations almost certainly won't represent as large a share of the electorate in those years, but the direction of the change is unmistakable -- and irreversible.

If current trends hold, that poses a distinct long-term threat to the Republican Party. On issues from immigration to gun control, and crime to transgender rights, Trump has unreservedly identified the GOP with the cultural priorities and racial anxieties of the portions of America -- primarily older, blue-collar, evangelical and rural whites -- most uneasy about the seismic changes remaking the nation.

One consequence of that choice has been widespread rejection of Trump among millennials. In 2016, Trump found an audience among non-college-educated white millennials that helped him win the Rust Belt states that tipped the election. But the latest CNN poll, conducted by SRSS , put Trump's approval among millennials last week at just 22%. A national Quinnipiac University survey released last week likewise found that about 7 in 10 millennials consistently expressed negative views about Trump across a wide range of questions.

Polling of the post-millennial generation remains much more sparse because pollsters don't survey teens nearly as often as young adults. But the polling that is available suggests Trump's version of the Republican Party faces at least as many obstacles with them.

One source is the huge annual survey of incoming first-year college students conducted by the Higher Education Research Institute at the University of California at Los Angeles. The survey of the 2016 nationwide college-entering class at schools across the country, the most recent available, offers a snapshot of the attitudes of the oldest post-millennials.

In that poll, 80% of the entering students said the federal government should address climate change, 71% said the wealthy should pay higher taxes and 68% said gun control laws should be more strict -- all priorities that Trump and congressional Republicans reject.

Also in the survey, 36% of the entering students identified themselves as "far left" or "liberal." That was a significant increase from the 24% that so identified in 1997, around the time the first millennials entered college. Since then, the share identifying as conservative or far right remained constant at just over 1 in 5; the portion identifying as middle-of-the-road fell from 55% to 43%. Among entering women, the study found, over 41% in the new poll identified as liberal or far left, the highest number recorded since the survey began in 1966.

The joint online study of people aged 15 to 24 by the Public Religion Research Institute and MTV conducted last July and August offers another vantage point. That study put Trump's approval rating among these young people at just 25% -- and considerably lower among the Hispanics (17%), African-Americans (5%) and Asian-Americans (6%) in that age group. Across a wide range of issues, these post-millennials and youngest millennials expressed cultural and racial views at odds with Trump's: 72% opposed building a border wall, 66% disagreed that efforts at increasing diversity usually hurt whites and 56% rejected the idea that Islam is inherently at odds with American values.

The poll did find a measurable but limited audience for Trump specifically and anxieties about racial change more broadly. His personal favorability rating among the survey's youngest respondents (ages 15 to 17) was slightly higher than among the older participants, but still very modest at just 29%. In the survey, 35% of white post-millennials said they approved of Trump, and a similar share of them supported building a border wall, agreed that encouraging diversity usually hurts whites and indicated that it bothered them when they came into contact with immigrants who did not speak English.

But in each case, the share of younger whites taking that position was much lower than among older whites; it was also considerably lower among younger white females than younger white males.

"I think one reason that white working class men were so attracted to Trump's message [in 2016] is that they felt betrayed by a society and culture that did not deliver for them what it did for their parents or grandparents," says Cox, the Public Religion Research Institute's research director. "Will young white men today have the same expectations about what society will be able to offer them? It's possible that white racial identity could be come more politically salient in the future, but there are yawning gaps between young white men and women, so it seems clear that it will be more complicated."

Of course, whites are a smaller share of the post-millennial generation than any other, so their views will be less dispositive in shaping its direction. Other changes cataloged in the poll underscore how great a change the post-millennials could bring. In the survey, 45% of these young people identified as religiously unaffiliated or non-Christian (compared with about 3 in 10 among all Americans), according to results provided to CNN. And about 1 in every 16 young men and 1 in 7 young women identified as gay or bisexual.

There's no evidence Democrats have cemented the loyalty of these young people. But those fundamental social attributes, like their racial diversity, create a more imposing gulf between the post-millennials and the Republican "coalition of restoration," which remains centered on the voters, and parts of America, least touched by demographic and cultural change . The student-led drive for new restrictions on gun ownership, particularly a ban on assault weapons, now opposed by Trump and almost all Republican elected officials, is only the first issue likely to map the depth of that divide.

"In some ways, the GOP also appears to be incredibly invested in the past, in maintaining traditional notions of American identity," Cox says. "How do you advance a belief that the US is a Christian nation without alienating the roughly 4 in 10 young people who are not religious? These are essential questions where there is no compromise or middle ground. These are opposing views about what America is all about."