Democratic Rep. Alan Grayson has announced his bid for U.S. Senate.

Grayson, who represented Florida's 8th congressional district from 2009 to 2011 before moving over to represent Florida's 9th district, is officially running for the Senate seat that will be vacated by Republican Sen. Marco Rubio, who is running for president.

"I'm Congressman Alan Grayson, and I would like to explain why I'm now a candidate for the U.S. Senate," the 57-year-old says in a video, the script of which was first provided to the Tampa Bay Times.

Grayson will now go up against U.S. Rep. Patrick Murphy and others for the Democratic nomination. Murphy, 32, announced Wednesday he has already raised $1.4 million in the quarter for his run. Murphy is a former Republican turned Democrat who defeated Republican Allen West in 2012.

"In 2012, and again in 2014, I was the only member of the House of Representatives who raised most of his campaign funds from small donors — not lobbyists, or special interests, or millionaires, or multinational corporations," Grayson says in the video. "I don't work for them; I work for you. I am unbought, and unbossed."

Grayson continues: "I wake up each day knowing that there are 700,000 Floridians who are counting on me to do something good for them in their lives. Next year, with the voters' support, that number will be over 19,000,000. I will do my best to help every one of us, whenever we need help. I will work hard to be your representative, your champion and your friend."

So far, four Republicans and four Democrats have officially declared their bids for the vacated Senate seat, which will be a crucial one for Republicans, who will be trying to keep control of the Senate.

Rubio, a Republican, was elected senator in 2010. His two predecessors, George LeMieux and Mel Martínez, were also Republicans.