IOWA CITY, Iowa The political fortunes of former Vice President Joe Biden and Vermont Sen. Bernie Sanders could hinge on how many young voters show up to next weeks Iowa caucuses , which are historically dominated by older voters with the ability to dedicate a full night to participating in the lengthy, labor-intensive process.

Polling in recent days has shown wildly divergent results for Sanders and Biden, who are competing with Massachusetts Sen. Elizabeth Warren and former South Bend, Indiana, Mayor Pete Buttigieg to win Iowa. The victor will receive a major boost to their efforts to claim the 2020 Democratic presidential nomination and the right to challenge President Donald Trump in November.

Age has emerged as the crucial dividing line ahead of the Democratic caucuses here, neatly dividing the coalitions of the two septuagenarians in a way education, class and geography do not. Sanders, whose bold agenda includes aggressive plans to wipe out student debt and battle climate change, disproportionately draws support from millennials and members of Generation Z. Biden, who has unveiled more cautious policy proposals, is heavily reliant on the gray-haired. Each is struggling with the opposite demographic . (The coalitions for both Warren and Buttigieg are more consistent across age groups.)

The Sanders campaign bluntly warned their chances of victory are reliant on college students, in particular, showing up to caucus.

None of these polls mean anything unless we actually achieve our goal of turning out a record number of college students on Monday, said Bill Neidhardt, the campaigns deputy Iowa director.

A New York Times/Siena College poll released Saturday morning found Sanders jumping to a substantial lead, earning 25% of the vote, while Buttigeg draws 18%, Biden 17% and Warren 15%. No other candidate earned double-digit support. John Anzalone, the pollster for the Biden campaign, immediately suggested the polls sample leaned too young. Nate Cohn, the Times reporter responsible for the poll, noted Sanders would have led even without the inclusion of any 18-to-29-year-old voters.

This sample has only 24% over the age of [65], Anzalone wrote. There could be up to 30% over the age of 65 going to the Iowa caucuses.

Meanwhile, a poll released last week from the advocacy group Focus on Rural America found Biden with a healthy lead, earning 24% of the vote to Warrens 18%. Buttigieg drew 16%, Sanders warranted 14% and Minnesota Sen. Amy Klobuchar 11%. Sanders allies have suggested that polls sample skewed older.

There are also contrary pictures of whether or not a surge of young voters could happen. A CIRCLE/Suffolk University poll found 35% of Iowans under the age of 35 planned to participate in the caucuses, which would amount to a massive surge from 2016, when just 11% of the same age group caucused.