The Note: What's Worrying Bernie Sanders?

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--BERNIE SANDERS MAKES IT OFFICIAL -- 'I AM RUNNING FOR PRESIDENT': Sen. Bernie Sanders, I-Vermont, is running for president in 2016, Sanders said in an interview with ABC's JONATHAN KARL. "I am running for president," Sanders said. "I intend to stand up and fight for working families all over the country." Sanders, who is the longest-serving independent in congressional history, said he will run for president as a Democrat. Sanders will hold a formal kick-off event in May, most likely in his home state of Vermont. Sanders' entry in the race gives Democrats at least one liberal alternative to former Secretary of State Hillary Clinton, who declared her candidacy earlier this month. Sanders will also be Clinton's first formal Democratic challenger, ABC's RICK KLEIN and ARLETTE SAENZ note. http://abcn.ws/1zlkKaz WATCH KARL's interview with Sanders on "Good Morning America": http://abcn.ws/1P9VN3u

--CONCERN ABOUT CLINTON: Sanders told ABC's JONATHAN KARL he is concerned by the millions of dollars flowing into the Clinton Foundation at a time when he thinks money plays too strong a role in politics. "It tells me what is a very serious problem," Sanders said. "It's not just about Hillary Clinton or Bill Clinton. It is about a political system today that is dominated by big money. It's about the Koch brothers being prepared to spend $900 million dollars in the coming election." "So do I have concerns about the Clinton Foundation and that money? I do," he added. "But I am concerned about Sheldon Adelson and his billions. I'm concerned about the Koch Brothers and their billions. We're looking at a system where our democracy is being owned by a handful of billionaires." http://abcn.ws/1EUjf5n

--MEET THE LIBERALS IN CONGRESS WHO ARE READY FOR BERNIE: Rep. Keith Ellison, D-Minn., and Rep. Raul Grijalva, D-Ariz., the co-chairs of the Congressional Progressive Caucus, didn't endorse Sanders in an interview with ABC News, but the two congressmen certainly endorsed the idea of their Capitol Hill colleague running, according to ABC's RICK KLEIN and ALI DUKAKIS. "We're proud that Bernie is going to make this a real race. He's going to bring up some important points, and we believe that it's a good idea to have a spirited discussion on these issues," Ellison said. WATCH: http://abcn.ws/1Fz7WOE