Former congressional candidate Jon Ossoff said Monday night that he's challenging Republican Sen. David Perdue in 2020 for a Georgia seat that could help determine control of the Senate. Ossoff, the fourth Democrat to join the race, announced his bid during an appearance on "The Last Word" on MSNBC.

"I'm running because we face a crisis of political corruption in this country," Ossoff told host Lawrence O'Donnell, calling Perdue a "caricature of Washington corruption."

Ossoff cited Congress not taking up gun control legislation and political influence at scientific institutions as examples of corruption.

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Perdue, a former Fortune 500 chief executive, has emerged as a close ally of President Donald Trump after being elected in 2014.

National Republican Senatorial Committee spokesman Nathan Brand called Ossoff an "unaccomplished, far-left candidate" in a statement.

Ossoff says he'll work to expand his network of supporters who helped him raise roughly $30 million in a 2017 special election he lost to Republican Karen Handel by about four points in Georgia's 6th Congressional District. At the time, the race had been the most expensive U.S. House race in history. Handel was defeated by Democrat Lucy McBath in the 2018 congressional election.

Perdue's isn't the only Georgia Senate seat that Ossoff could have run for. A special election to fill the remaining two years of Republican Sen. Johnny Isakson's term will also be on the November 2020 ballot, after Isakson announced his retirement late last month due to health issues.

The two open Senate seats have raised Georgia's status as a must-watch 2020 battleground.

While Ossoff will face a primary contest in the race for Perdue's seat, the race for Isakson's seat won't have any primary and will instead be open to all qualified candidates, significantly increasing the likelihood of a runoff.

Ossoff faced that type of wide-open race in his 2017 special election loss. He won the most votes by far in the 18-candidate race for the suburban Atlanta House seat in April 2017, but fell short of the over 50% needed for an outright win. Ossoff lost the subsequent runoff to Handel.

Ossoff, a 32-year-old investigative filmmaker, joins 2018 candidate for Georgia Lt. Gov. Sarah Riggs Amico, former Columbus Mayor Teresa Tomlinson and Clarkston Mayor Ted Terry seeking the Democratic nomination in the race against Perdue.

The field began to form after former Democratic candidate for Georgia governor, Stacey Abrams, announced in April that she would not run for the seat after being heavily recruited to. In a video released on social media, Abrams said that she is "committed to doing everything I can do hep elect a Democrat to that seat next year." Abrams also said that she would not be vying for Isakson's seat.

Brand said Ossoff's "extreme left-wing views will fit in with the rest of the crowded Democratic primary but will stand in sharp contrast to David Perdue's positive record of delivering results for all of Georgia."

Ossoff says on a newly launched website that he's "building a movement to mount an all-out attack on corruption in Washington — starting with President Donald Trump and Georgia Senator David Perdue."