The biggest moment of the 2016 campaign so far has arrived. After hours of speeches, shaking hands and kissing babies, both presidential nominees will engage in their first debate tonight. Millions of undecided voters will turn on their televisions and make their choice after watching how the candidates perform.

I say performance because a debate is more than just an exchange of ideas. Everything Hillary Clinton and Donald Trump say and do will be scrutinized on those three nights, so they have to prepare carefully.

A gaffe could easily sink a campaign. During the 1976 presidential election, incumbent Gerald Ford famously claimed that there was “no Soviet domination of Eastern Europe” under his administration. In a very close election, this gaffe probably propelled Jimmy Carter to the White House.

Sometimes the mistake can be from behavior. During a debate in 1992, President George H.W. Bush consistently looked at his watch as if he wasn’t interested in debating about important issues. In 2000, Democratic nominee Al Gore annoyed the audience with his continuously loud sighing at a debate. It even could just be appearance, as was the case with Richard Nixon and John F. Kennedy at their 1960 debate.

Donald Trump and Hillary Clinton, both historically unpopular for different reasons, have the opportunity to let voters view them in a different light. Clinton may want to improve on how voters view her personality — giving a joke at the debate can go a long way. She will also need to be prepared for questions relating to her health and the email scandal. Those issues are her biggest flaws. As for Trump, it’s all about his knowledge of specific issues and his policy solutions.

Since her health scare, Clinton has slightly improved in the polls. As of Sept. 25, an average of national polls shows her up by a margin of 2.8 percent. Even so, the margin is too narrow for her to feel comfortable, and she’s led him by higher margins at other points in her campaign. There are also higher expectations of Clinton than Trump. She is an experienced politician who has been on the debate stage numerous times in previous elections. Voters expect a stellar performance.

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Trump’s biggest problem has been that he’s a gaffe machine. During the nomination battle, he regularly insulted his fellow candidates as well as many in the media. After the convention, Trump’s poll numbers collapsed when he attacked the Khans, a Muslim family who had lost a son in Iraq and were given a speaking slot at the Democratic convention. Trump made the mistake of disrespecting the family, which immediately backfired and proved he couldn’t handle criticism.

Recently, Trump has been avoiding scandal and has tried to build some outreach to the African-American community. But even with these improvements, he can’t just ramble about a far-fetched plan he has concocted or act rude. More voters are watching him now than during the Republican primary debates, so he has to behave himself.

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The candidates are getting ready for their confrontation. They both know the power of the media. How they act during these debates will give them new momentum or sink their campaigns. More than anything else, the debates will let the voters see two visions for the country. In November, they will choose which version they want to live in for the next four years.