"Millennial voters could play key role in presidential race,” declared a recent Newsday headline for an article touting "the demographic shift to a younger electorate." The article is typical of reports that millennials—the generation born from the early eighties to the early aughts—have pulled even or even surpassed boomers as the largest eligible group of voters. This sort of article obscures a crucial fact: Seniors make up an increasing, not decreasing, share of Americans eligible to vote. According to the Census Bureau's National Population Projections, both the 50-64 and 65-and-over age groups are growing as a share of all eligible voters. And they will continue to grow into the 2020 and 2024 election cycles. The senior vote represents the future, not the past.

Which is why this year's U.S. elections may end up surprising pundits in the same way that the Brexit referendum defied predictions in the U.K. Seniors there turned out in significant numbers in June and voted by large margins for Britain to leave the European Union. They overwhelmed younger voters and minority voters, demographics that favored remaining in the EU.

Similarly, data suggest that seniors have had more impact in recent U.S. elections than they are given credit for and will likely do so again this year. America's seniors, not Hispanics or youth, are the key to Republicans winning the presidency and maintaining a majority in Congress.

Claims for the impact of young voters often point to the number of youth eligible to vote. But predictions based on the preferences of eligible voters mean little. It's only those who actually show up at the polls who count. And seniors are the most reliable voters—about 70 percent of them can be counted on to cast ballots. Compare that to 18-24-year-olds, who have only exceeded a 40 percent turnout twice since 1996. In election after election, the older you are the more likely you are to vote. As Pew Research reports, only 10 percent of likely voters are under 30; more than two-and-a-half times as many likely voters are over 65.

Hispanics, as voters, make their numbers felt even less. The 2014 election was typical. Although voting in midterm elections is always less than in presidential election years, the relative numbers are telling: While 45.8 percent of white and 40.6 percent of black eligible voters turned out two years ago, only 27 percent of Hispanics did. Of the 25 million Hispanics eligible to vote, only 6.8 million showed up at the polls.

Thus, if you factor in both demographics and reliability, a much more realistic picture emerges. Those 55 and over will likely make up no less than 44 percent of all votes cast this year. Those 18-24 will account for less than 10 percent, and Hispanics even less than that.

But national numbers are only part of the story.

It's no secret that the outcome of the presidential election and control of the Senate will come down to a few battleground states. Here too seniors are increasing as a share of eligible voters. In Ohio, North Carolina, Florida, Illinois, Arizona, Wisconsin, and other key states, seniors will represent on average 2.5 percent more voters than they did in 2008. If current trends continue, no less than 46 percent of all votes cast in these key battleground states will come from those 50 and older.

The news gets even worse for Democrats because seniors are realigning with the Republican party in record numbers. In 1992, according to Gallup, 53 percent of senior citizens identified as Democrats and only 39 percent called themselves Republican. By last year the GOP had pulled ahead with seniors, with 48 percent of them self-identifying as Republican and 45 percent as Democrat.

Although many political pundits and the national media repeatedly stress the important role millennials and Hispanics will play in the outcome of this year's elections, scant attention is paid to seniors. That error can lead campaigns to make mistakes in targeting their resources. This isn't just a matter of going after the wrong voters, it can also be a matter of going after them in the wrong way.

Last year I wrote in The Weekly Standard that seniors "may not be tech-savvy or trendsetters, but are a political powerhouse" ("Remember Who Shows Up to Vote," August 3, 2015). Since then, in an April 2016 national survey of registered voters, Nielsen/Scarborough found that despite all the focus on online and digital media only 51 percent of voters between 50 and 64 years old own smartphones, and only a quarter of those over 65 have smartphones. New-media applications have their limits when half of the largest, most dependable voting demographic doesn't have smartphones.

When election results don't turn out as expected, the first reaction is often to question the accuracy of the political polls. But maybe, as we recently saw in the U.K., the real reason is that the senior vote is overlooked and underestimated.

This year, it will be a huge mistake if political operatives and the media sell short our nation's seniors. They vote.

Tom Edmonds is a veteran Republican political media consultant. He is a past president of the International Association of Political Consultants.