(CNN) The sharp turn against the Republican Party by young people in the 2018 election may be only the overture to an even greater political risk for the GOP in 2020.

Both historical voting patterns and underlying demographic trends suggest that the biggest difference in the electorate between this election and the next one is that relatively younger voters will cast a greater share of the votes in the presidential year -- perhaps a much larger share. Even with much higher than usual turnout among young voters this year, voters 45 and below are likely to increase their proportion of the total vote from just under three-in-ten this year to something closer to four-in-ten by 2020, historical trends suggest.

"They will certainly be a larger percent of voters than they were in 2018 given presidential versus midterm trends," says Yair Ghitza, the chief scientist at Catalist, a leading Democratic voter targeting and election modeling firm. "The question is to what extent the [higher] engagement we saw in 2018 will continue and be better than in 2016 and other presidential years."

A rising participation level could threaten Republicans at a moment when younger voters, who have consistently expressed preponderant opposition to President Donald Trump in polls, provided Democrats their largest margins in decades during last month's election.

"Voters under 45 moved decisively and overwhelmingly toward Democrats, and I don't know how you take it as anything other than a total rebuke of Trump and what's he done," says Democratic pollster Andrew Baumann, who has extensively studied younger voters.

Despite Democrats' emphatic gains among younger voters, Republican pollster Kristen Soltis Anderson, author of The Selfie Vote, a book on the Millennial Generation, says the GOP shows no signs of grappling with the shift. "Even though the election, especially on the House side, was not good for Republicans there has not been an appetite for a course correction or a change in approach," she says. "So it would surprise me if there was a concerted effort to try win over more young voters between now and the 2020 election."

Methodological changes this year in the exit polls conducted by Edison Research for the National Election Pool, a consortium of media organizations that includes CNN, make it impossible to compare the share of the vote cast in 2018 by voters under 45 with that in earlier years. Census Bureau data on the electorate's composition isn't available yet.

But preliminary Catalist analysis of the 2018 results provides a consistent data source to assess the trends. Past numbers from Catalist are based on actual turnout tied to voter records while it generated this year's preliminary numbers by comparing their pre-election data on registration, early voting and planned turnout with actual results at the precinct or county level. The analysis found that both voters aged 18-29 (the younger half of the millennial Generation) and those aged 30-44 (the older millennials and younger part of Generation X) constituted a larger share of the vote this year than in 2014, the most recent mid-term election. Combined, those voters under 45 cast 29% of the votes last month, Catalist estimates, compared to 26% in 2014.

Several states with Democratic campaigns that particularly targeted young people saw bigger increases, according to previously unpublished Catalist data. In Arizona, the share of the vote cast by those under 45 spiked from 21% in 2014 to 29% this year; in Georgia, the numbers jumped from 29% to 36% ; Texas increased from 26% to 33%.

Expect more young voters in 2020

All of that reflected the unusually high level of engagement among young people this year: Tufts University's Center for Information on Civic Learning and Engagement, which studies younger voters, estimates that about 31% of those 18-29 voted in 2018, up from only about 20% in the 2014 mid-term.

But, critically, even with these gains, young people still constituted a smaller share of the vote than in the recent presidential elections, both nationally and in the key states. In Catalist's estimates, voters under 45 represented 36% of the voters in 2016, while the Census Bureau put the figure at 38%. In each case that is well above Catalist's 29% for 2018. In the states, Catalist calculated that the under-45 share of the vote this year still substantially lagged behind the 2016 level not only in younger Sun Belt states such as Arizona, Georgia, Florida, North Carolina, and Texas, but also in older Rust Belt states including Ohio, Michigan, Wisconsin and Pennsylvania.

That raises the clear red flag for Republicans that the young people who broke decisively against the party last month are likely to comprise a measurably larger share of voters in 2020.

The most basic reason younger voters loom even larger in 2020 is that the millennial Generation and Generation X, joined for the first time by the post-millennials born after 2000, will comprise a larger share of the eligible voter pool than in 2016. (Those three generations represented about 55% of all eligible voters in 2016, and are expected to rise to about 63% in 2020, according to forecasts by the non-partisan States of Change project .)

Millennials are getting more engaged, but they aren't becoming Republicans

The participation of these groups could also rise because all generations vote in greater numbers as they age. And their numbers could swell further if they maintain the heightened enthusiasm evident in 2018. "I don't really see any scenario where young voters retrench next cycle," says Baumann. "The question is whether our [presidential] ticket can maximize that."

Heightened turnout in 2020 would raise the price for the losses Republicans suffered among younger voters this year. In the exit polls, Democrats carried fully 67% of voters aged 18-29 in House elections. That represented their best performance among adults under 30 in any House election since at least 1986; it even exceeded their modern high points of around three-fifths in the 2006 midterm election and the 2008 and 2012 presidential years, when former President Barack Obama was on the ballot. Just two years ago, House Democrats carried a much less imposing 56% of these voters in the 2016 election, exit polls found.

Similarly, exit polls this year found House Democrats captured 58% among voters aged 30-44. That's also the highest share of the vote Democrats have won in that age group since 1986. House Democrats had lost those voters, who might be described as early middle-aged, as recently as 2010 and had not carried more than 52% of them in any of the three elections since.

These results were remarkably consistent across regional lines. Democrats carried voters aged 18-29 in all 22 Senate races in which an exit poll was conducted, except for Indiana, where the two candidates tied. Many of their margins among these youngest voters were enormous. In the US Senate race in Texas, Democrat Beto O'Rourke, despite losing the race, carried 71% of voters younger than 30, the exit polls found. Gavin Newsom won 69% of voters younger than 30 in winning the California governor's race, and Stacey Abrams carried just under two-thirds of them in her losing Georgia gubernatorial bid.

Democrats also won voters aged 30-44 in every Senate race with an exit poll except for North Dakota, although the margins in some states were considerably narrower than with the youngest voters. Anderson says that weak showing among voters in early middle age should be especially alarming for the GOP. "When you look at these crosstabs, and you see just how poorly Republicans did among thirty-somethings, not just kids just out of college, it's a problem," she says. "They have kids, they bought houses, they pay taxes, they are doing all of those things that were supposed to make them Republicans, and they didn't become Republicans."

To Anderson, the GOP's weakness with voters now in their thirties is evidence that the party's problem extends beyond Trump: as she notes, the roughly three-fifths of voters 30-44 that Democrats won in 2018 almost exactly equaled their showing among voters in their twenties during their sweep in the 2006 mid-term election. "The problem now for Republicans is you are not just talking about we need to do better with younger voters, now it has spread so far up the age scale as these voters have gotten older and not become more conservative in the process," she said. "This a problem that emerged in the mid 2000s that was never really properly addressed, and it's now taken much more root."

Trump hasn't helped the GOP with younger voters, but Clinton didn't really help Democrats

But while the GOP's difficulties with the Millennial Generation predate Trump, there seems little doubt that he has compounded them. From the outset, many millennials viewed Trump's belligerent language on race and immigration, and his belittling comments about women, as an explicit counterrevolution against the ideal of a more inclusive and tolerant America that most of them say they support. In a summer 2016 ABC/Washington Post survey, two thirds of voters under 40 said they considered Trump biased against women and minorities.

But doubts about Democratic nominee Hillary Clinton blocked the full expression of those doubts: while Trump in 2016 carried almost exactly the same share of voters under 30 as GOP nominee Mitt Romney did in 2012 (just over one-third), Clinton fell a crucial five percentage points below Obama's showing, as many young people scattered to the minor party alternatives. Voters 30-44 split even more closely, with Clinton carrying just 51 percent of them in the exit poll. (Catalist's vote modeling produced generally similar results for both groups in 2016.)

In this election, Trump faced a withering verdict from younger voters. In the exit polls, 66% of voters aged 18-29 and 62% of those aged 30-44 said they disapproved of his performance in office. In each group, just over half said they strongly disapproved of Trump's performance, significantly more than the share of older voters (just over two-fifths) who said they were so strongly disenchanted with him.

"The disapproval of Trump, and the views of him as being a racist and sexist that we saw [among young people] in 2016, was somewhat muted by not loving Hillary Clinton," said Baumann. "But it just got amplified after him being in power for two years. One of my theories coming out of 2016 was that Republicans by embracing Trump were at risk of losing a generation of voters, and it sure seems like that is coming to the fore now."

Strategists in both parties agree Trump could survive greater rejection from younger voters in 2020 if he inspires high turnout and big margins among his core groups of older, non-urban and blue-collar whites; those groups are especially prevalent in the Rust Belt states that keyed his victory last time. And young people from rural areas or white working-class backgrounds have shown much greater openness to Trump's message than their generational counterparts who are non-white, college-educated or living in large metropolitan areas.

The size of Trump's base is shrinking

But Trump is still committing the GOP to a strategy of squeezing more advantage from groups that are shrinking. All of the major data sources on the electorate's composition -- from the Census Bureau to the exit polls to Catalist -- agree that the share of the vote cast by Trump's core group of whites without a college education has been declining by about two percentage points over each four-year presidential cycle. With turnout among minorities and college-educated whites surging, Catalist's preliminary analysis found those working-class whites, while still the electorate's largest single group, dropped fully five points as a share of the vote this year, compared to the last mid-term in 2014.

One thing no political strategy can reverse is the tide of generational replacement. As not only the World War II and Silent Generations, but also more baby boomers pass out of the electorate, the share of the eligible voting pool comprised of Generation X, millennials and Post-millennials is inexorably rising. The States of Change project forecasts those three generations -- which are much more racially diverse and college-educated than the generations they are replacing -- will continue growing to about two-thirds of eligible voters by 2024 and nearly three-fourths by 2028. More voters mean more consequences if Republicans can't soften the recoil from the party that younger voters displayed last month. "It's...a clear and present danger to any Republican policy maker who expects their name will be on a ballot after 2020," says Anderson. "Donald Trump will run for reelection in 2020, and I am out of the business of saying he can't pull this off. But if you are a more conventional Republican...who has ambitions of being in political office for more than the next two years you ought to be gravely concerned about this."