Bernie Sanders is a democratic candidate who has shown that he can raise the money needed to run for president. His supporters are individual citizens who believe in his message and are willing to give him money to fund his campaign. The people are voting with their pocket book and that is what makes Bernie Sanders a serious contender.
A large value for monetary contributions does not mean that he has received the support of many voters, it only shows that he has the support of at least few wealthy supporters. While the money can be used to help gain more supporters through advertisement it does not in and of itself mean that he is a viable contender for the presidency.
I believe that Bernie Sanders is a serious contender for president. However, just because he has received over 2 million contributions that does not make him a more serious contender than Hillary Clinton. Bernie Sanders claims to be a socialist and not Hillary Clinton. Hillary Clinton has values that are more closely aligned with the Democratic Party in America. There is a lot of support for Mr. Sanders but I think Hillary is more well know and runs closer to her party's line than Bernie Sanders.
In spite of landing over 2 million campaign contributions, Bernie Sanders is not a serious threat to surpass Hillary Clinton in the poles. As Secretary of State and former First Lady of the White House, Clinton has been a mainstay in American politics for more than twenty years. On the other hand, until 2015, Sanders was only a household name in Vermont, the state where he once held office.