The crowds that marched on the White House Sunday in opposition to the Keystone XL Pipeline project arrayed themselves behind a banner that read, “We did NOT vote for KXL.”

That was the most vital political message of a day that saw almost 400 Americans—the overwhelming majority of them young people—arrested as part of a dramatic protest against the oil pipeline project that has drawn outspoken opposition from environmental groups.

A lot of Washington politicians, pundits and professional strategists miss the political dynamic that goes with the pipeline debate. Polling shows that young people “get” the climate change issue, and that they see it as a high political and personal priority.

Indeed, they care about it so much that they marched on the White House to urge the Obama administration not to approve the Keystone proposal. Hundreds were willing to be arrested. They recognize, as notes Smith College student Aly Johnson-Kurts, an organizer of Sunday’s protest, that “the traditional methods of creating change are not sufficient…so we needed to escalate.”

This notion that traditional methods of creating change are not sufficient is significant, especially for Obama and his party.

The Democrats have relied in recent presidential election years on overwhelming support from voters under the age of 30. And they have suffered as enthusiasm among young voters has declined in off-year congressional elections.

In 2008, exit polls suggested, voters aged 18–29 accounted for 18 percent of the 131,313,820 Americans who turned out. Obama won their votes by a striking 66-31 margin over Republican John McCain. Obama’s winning margin was roughly 10 million votes, of which more than 7 million came from young people.

In 2012, according to exit polling, younger voters increased as a percentage of the overall electorate, with 18–29-year-olds making up 19 percent of the 129,085,403 who turned out. They favored Obama by a 60-36 margin. That translates to an advantage of more than 5 million votes for Obama. Notably, Obama won the national popular vote by 4,982,296 votes.

There are analyses that suggest an even more significant youth-vote benefit for Obama and the Democrats in battleground states. But the national numbers should establish the importance of the youth vote.

Unfortunately, turnout among young people tends to slide in off-year congressional elections—like the critical one that the US faces in 2014. In 2010, when Democrats suffered serious setbacks at the federal and state levels, voters under 30 made up just 11 percent of the overall electorate. They still backed Democrats—indeed, they were the only age demographic to do so—but their ability to influence election results was reduced by the sharp reduction in numbers.

The Obama administration must make its call regarding Keystone based on science and sound long-term thinking regarding energy, environmental and agricultural policy.

But those who talk about the political ramifications of this decision should keep in mind that sign that read “We did NOT vote for KXL.”

A 2013 poll found that more than 60 percent of young Americans felt that, were the administration to approve the pipeline, Obama would be breaking a campaign promise. And a significant percentage of those surveyed said they would feel betrayed by a decision to let the Keystone project go forward.

If young voters get a signal that they are not being heard, if they feel disappointed and disenfranchised, there is every reason to believe it will be harder for Democrats to mobilize them in 2014.

That does not mean that all young voters will stay home. Younger voters are not single-issue voters. Millions will still go to the polls in 2014, including, undoubtedly, the vast majority of those who marched on Washington Sunday. But if their percentage of the overall electorate is low, and if a portion of those who do turn out opt out of frustration or hope for a Green alternative, an already tough election season could get dramatically tougher for the Democrats.