The moment former Secretary of State Hillary Clinton mentioned a possible run at the presidency, the 2016 Democratic nomination was determined for most. But over the last year, Vermont Senator Bernie Sanders has made his voice heard and is giving Clinton supporters some major doubts.

Sanders’ campaign has gained the support of millions of people across the country. Now, as we get closer to the first set of primaries, what looked impossible is shaping into a scenario that could lead to one of the biggest upsets in political history.

A national poll released by Monmouth University on Dec. 16 showed Clinton is still up on Sanders by 33 points. But it is just a national poll; the nominations are won state by state. For Clinton to gain momentum, she will need to win the first two states: Iowa and New Hampshire.

Iowa will be the hard one for Sanders to win. Still down in the polls as of December, Sanders will need to close the gender gap between him and the substantial amount of women supporters Clinton has if he hopes to be successful. According to another poll from Monmouth University as of December 2015, Clinton was up, 2-1, among democratic women.

Iowa is a caucus state, however, meaning candidates aren’t selected by popular vote. Instead, a caucus is where registered voters from each precinct of the state come together in various locations to debate on behalf of the candidate they prefer. Whichever candidate gets the most support ultimately wins the state. If Sanders is strong during the caucuses and is able to get the right people in positions at the delegate elections, there is a definite possibility of him winning.

Right now, Sanders is making a real push, especially with his recent major union endorsement from the Communications Workers of America. With this just occurring, the effects it could have in the Iowa caucus are significant. With New Hampshire following and the senator already hosting a five-point lead in his home state of Vermont, the momentum Clinton had could vanish.

“If we can win in Iowa, if we can win in New Hampshire, we have a real path toward victory, to pulling off one of the major political upsets in the history of our country,” Sanders said to about a hundred students at Nashua Community College, according to The National Journal.

Only one president has ever won the election after losing the first two primaries; coincidentally, it was Bill Clinton.

Bernie Sanders has sat on the back burner of this election for some time now. With all the attention going toward Clinton and Trump, Sanders has been able to climb his way up the ladder and make a real mess of things for people who were certain Clinton would be the Democratic candidate.

There are just a few more weeks until the race really begins, and with Sanders gaining momentum each day, Clinton’s long-projected run could come to a sudden halt.