Bernie Sanders' 2016 presidential campaign is revving up, and as he continues to gain support, Hillary Clinton might actually find Sanders to be a legitimate rival for the Democratic nomination. The Vermont senator is a self-described democratic socialist, which scares many voters away, but not enough to make him a trivial candidate. In fact, it could work in his favor. A new Gallup poll found that Democrats and Democratic-leaning independents are more liberal than in the past — 47 percent identify as both socially liberal and economically moderate or liberal, up from 39 percent in 2008. This is excellent news for Sanders.

Clinton is still the Democrats' favorite for the party's nomination, but that doesn't mean she can sit back and relax. In 2008, a little-known senator from Illinois came out of nowhere and won the nomination and the presidency (you may know him as President Obama). Although Clinton has less competition this time around, she should still be worried about the past repeating itself. Sanders is quickly gaining name recognition, and he has a few key advantages over Clinton, including his lack of controversial scandals — of which Clinton has a few.

Here are four reasons Clinton should be worried about Sanders in the 2016 election.

He's Drawing Big Crowds

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As Sanders' campaign picks up speed in preparation for the February caucuses, his events are attracting more and more people. His Minneapolis rally at the end of May had a crowd of more than 3,000 and an event in Denver Saturday drew 5,000 supporters. If the numbers continue to increase, Hillary could have a real problem.

He's Has A Clean Past

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So far in the race, the only dirt that's surfaced on Sanders is a rape fantasy essay he wrote in 1972, but even that was quickly forgotten about. Clinton on the other hand, has a fairly long list of past scandals, including "Emailgate," the Clinton Foundation accepting money from Middle Eastern governments, and the Benghazi attacks.

He's Raised A Lot Of Money

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Out of the 2016 candidates who've revealed how much money they raised on their first day, Sanders raised the most — a whopping $1.5 million in 24 hours. Marco Rubio followed closely behind with $1.25 million. In his first four days as a candidate, Sanders raised $3 million for his campaign. Though Clinton has not disclosed how much campaign money she's raised thus far, it's probably a lot, but Sanders has proven that he can compete.

He's More Green

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Sanders has been dubbed the "best candidate on climate change," an issue that liberals are increasingly worried about. A recent NBC News/Wall Street Journal poll found that 15 percent of Democrats think climate change should be the top priority for the federal government, outranked only by economic growth and health care. Sanders could really use his history with creating and voting for green legislation to his advantage. Watch out, Hill.

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