A dozen presidential candidates will face one another in Ohio on Tuesday at the fourth Democratic debate, where the three front runners—all of whom were born in the 1940s—must hold their ground against a younger, more diverse set of challengers.

Former Vice President Joe Biden, 76, Vermont Sen. Bernie Sanders, 78, and Massachusetts Sen. Elizabeth Warren, 70, are the ones to beat. The three septuagenarians have been leading state and national polls for months, but all face challenges in a race playing out in the shadow of an impeachment drama on Capitol Hill.

Biden, the most persistent front runner, has faced an onslaught of questions in recent weeks about his son Hunter Biden's allegedly unethical business dealings abroad. (The allegations, which have been fueled by President Trump himself, are unproven.) Sanders's heart attack on Oct. 1 has raised concern among both his supporters and critics about his physical fitness for office and Warren has embroiled herself in an insider-y war with Facebook over its practice of promoting untruthful political ads.

A raft of younger contenders, armed with a patchwork of policy solutions and soaring rhetoric about national unity, are hoping to find a chink in that gray wall—and Democratic political strategists predict some movement at the top before the Iowa caucuses kick off next year.

One of the biggest wild cards is how Biden will weather questions about Hunter Biden’s deals in Ukraine and China, which damage the former Vice President's image as the most likely to be able to take on the President in a general election. Trump, who benefits if Biden's star fades, has redoubled his public attacks on the Biden family's alleged wrong-doings in recent weeks, in much the same way that, in 2016, he latched onto Hillary Clinton’s use of personal emails. (The FBI declined to prosecute Clinton, but the whiff of scandal lingered throughout the campaign.)

If Biden appears weak, South Bend Mayor Pete Buttigieg, 37, as well as Senators Kamala Harris, 54, Amy Klobuchar, 59, and Corey Booker, 50, stand to gain. All four appeal to moderate Democratic voters who, if Biden stumbles, may be in search of a new horse.

Sanders' health will also be scrutinized at the debate. If the Vermont Senator appears shaky on his feet, supporters may begin looking for a new candidate. Warren will likely benefit directly from such Bernie-defectors, but a handful of other candidates may see their numbers tick up too. Like Sanders, Businessman Andrew Yang, 44, and Rep. Beto O’Rourke, 47, have run slightly quirky, outsider-y campaigns with wide appeal.

Former Housing and Urban Development Secretary Julian Castro, Rep. Tulsi Gabbard and businessman Tom Steyer, all of whom are polling at 1% or below, must deliver performances on Tuesday that allow them to stay on the debate stage at all.

Warren, who has run a strong campaign thus far, risks alienating more centrist, socially-conservative voters in the crowd on Tuesday. While the Democratic base tends to be more liberal than the general electorate, the location of the debate—the affluent suburbs of Columbus—may mean a slightly more traditional audience. On Saturday, Warren drew criticism from Republicans when she glibly dismissed a hypothetical question from a voter who believed marriage is between a man and a woman. ("Then just marry one woman. I’m cool with that," she responded, before adding, "Assuming you can find one.")

The debate, which is hosted by CNN and the New York Times , will take place in Westerville, Ohio, in the suburbs of Columbus—exactly the type of conservative place that Democrats hope to turn blue in 2020. While Republicans have traditionally dominated affluent suburbia, polls show that the white, college-educated voters, and particularly women, may be unwilling to vote for the party of Trump.

Westerville, for example, saw an almost-13 point swing in Democrats’ favor between 2012 and 2016, with Clinton besting Trump by 4.4 percentage points. Clinton, in other words, won in a city that Barack Obama tried and failed to win in 2008 and 2012. The Democratic nominee for the U.S. House race last year came up short in a gerrymandered district, but Democrats across the ticket out-paced Republicans 56% to 43% in Westerville proper.

While it's still early in the race—Election Day 2020 is more than a year away—all twelve Democratic candidates must jockey for the top spots now. While the debate Tuesday will be the biggest in history, only a handful will be left standing at the podiums by spring.