When Senator Barack Obama was elected in 2008 with a majority vote over Republican Senator John McCain, he was heralded as a possibly transformational President -- potentially in the same category as Franklin D. Roosevelt and Ronald Reagan. I know that argument well. I was one of the pundits making the case. It seemed very simple then: like his two iconic predecessors, Mr. Obama's election created a new demographic coalition, one so radically different than before and one that would surely dominate the national landscape for at least a generation. Not only did he receive the solid majority of Hispanics, African Americans, and Asian Americans, but he was dominant among younger voters and the growing - strategically located - Creative Class.

Mr. Obama's majority triumph in 2012 only reinforced the argument that his was the coalition of the future. But his problem soon became very clear: unlike FDR and the Gipper, President Obama seemed unable to turn his triumphant election coalition into a successful governing coalition. He was not able to rally voters to his side as he pushed for his Affordable Care Act, let alone build majority support for infrastructure renewal, environmental regulations, and much else of his domestic agenda. Unlike President Lyndon Johnson, he was not able to turn his victories into a posture that allowed him to bully members of Congress from either party.

His reward: an almost record loss of seats in the 2010 elections to the House of Representatives and the election of Tea Party representatives who, let's just lay it on the line, simply hate his guts.

History will ultimately judge whether Mr. Obama is enumerated among the transformational Presidents. No doubt, America's first African American President will be memorialized in brick and mortar, along highways, in stone, and etched in coin. Make no mistake about that. We will have to wait for a lot of dust to settle - a lot of dust - before we see if the Affordable Care Act made people's lives better or worse; the same for whether or not his stimulus programs actually stemmed or worsened the Great Recession, or his executive actions (sans Congress) reduced carbon emissions or improved the lives of women and gays.

But for the time being, let us consider whether Mr. Obama's electoral victories were just the result of a charismatic figure or if they warrant his being labeled at least a "transformational candidate". Has the national electorate truly changed? Is his victory coalition long-term or just a passing fancy? In my view, it all depends on simply one group - Millennials (First Globals I call them), those Americans born between 1979 and 1996. Millennials have increased their share of the national presidential electorate substantially in both 2008 and 2012. But they fell short with a very low turnout in 2010. Exit polls show that 18-29 year olds were about 19% of the total votes in the presidential elections, but only about 10% in 2010. Hispanics and African Americans went from 9% -10% and 13% respectively in the presidential years but represented merely 6.6% and 10.9% respectively in 2010.

As I look at the U.S. Senate races this year, I see polls right now in 8 states showing margins between candidates at 3 percentage points or less. If Millennials were to increase their average turnout in these races from 10% to just 14% or 15%, a matter of about 30,000 to 70,000 voters in most states, this would change the dynamic in these states from "leaning GOP" or "slight lean to Democrats" to decisive Democratic victories. As I look at recent polls by Zogby Analytics, I find that 18-35 year olds - the Millennials - are the group most likely of any age group to say the U.S. is headed in the right direction, to choose Democratic candidates in the congressional generic ballot, to approve of the job that President Obama is doing as President, and to feel optimistic about their personal finances in the next four years. They are also the age cohort least favorable to the GOP. An increase in turnout of Millennials means a higher percentage of Hispanics and African Americans.

In short, the 2014 elections do matter. History will judge pro or con on the efficacy of Barack Obama as President but this year's elections will determine if he has truly set in motion a profound and lasting demographic shift in the American electorate. Millennial hold the balance.