Story highlights Millennials will be the largest voter-eligible group in the 2018 midterms

This steady shift in the electorate's composition should benefit Democrats

(CNN) In 2018, the American electorate will cross a historic threshold that could reshape the political balance of power-or leave Democrats fuming in frustration at continued Republican dominance of Washington.

For the first time, millennials next year will pass baby boomers as the largest generation of Americans eligible to vote, according to the well-respected demographic forecasts from the States of Change project at the Center for American Progress, a liberal advocacy group. That transition will end a remarkable four decades of dominance for the baby boomers, who have been the largest generation of eligible voters since 1978, when they surpassed what's been popularly referred to as the Greatest Generation (or G.I. Generation) raised during the Depression.

This steady shift in the electorate's composition should benefit Democrats. The baby boom, which is predominantly white, has drifted reliably toward the GOP in recent years: both Mitt Romney and Donald Trump carried about three-fifths of whites older than 45, exit polls found. By contrast, when compared to older generations, the millennial generation, which is more racially diverse and less religiously devout, votes more often for Democrats, expresses more opposition to Trump and professes more liberal views, particularly on social and environmental issues.

But the impact of this change will be muffled if millennials continue to vote at much lower rates than their elders, particularly in midterm elections like 2018. Though millennials are on track to soon surpass baby boomers as the largest generation of potential voters, experts caution that because of their lower turnout it may still be years until they exceed their elders among actual voters. With polls now consistently showing a huge generation gap over Trump in particular and the parties in general, the next several elections could hinge on how fast millennials convert their potential influence into actual power at the ballot box.

The long-term electoral shift from the baby boom and older generations toward millennials is unmistakable. The first millennials -- generally defined as the generation born from 1981 to 2000 -- entered the electorate in 2000. At that point, according to Census figures analyzed by States of Change, they represented just four percent of eligible voters; baby boomers (born between 1946 and 1964) constituted nearly 10 times as many eligible voters, at 39%. By 2016, the two generations had virtually converged, with Census figures showing that baby boomers represented just over 31 percent of eligible voters, and millennials just less than 31%. (In the process, millennials surged past Generation X, Americans born between 1965 and 1980, who comprised about one-fourth of eligible voters last year.)