Maryland Sen. Ben Cardin, the ranking member on the Senate Foreign Relations Committee, filed for re-election on Monday.
He will face four other Democrats, including Chelsea Manning, who was convicted of sharing military documents with WikiLeaks, as well as Jerome Segal, Richard Vaughn and Debbie "Rica" Wilson.
Segal is a research scholar at the Institute for Philosophy and Public Policy at the University of Maryland, Vaughn is a businessman and Wilson is a former public school teacher and college professor.
Cardin was a primary sponsor on the Countering Russian Influence in Europe and Eurasia Act of 2017, which was incorporated into a bill imposing sanctions on Russia and signed by President Trump in August, according to GovTrack.
The law included a Jan. 29 deadline to impose new sanctions, but the Trump administration let it pass.
In June, Cardin introduced the Chesapeake Bay Program Reauthorization Act, an environmental restoration effort.
The youngest elected speaker of the Maryland House of Delegates, Cardin served from 1979 to 1986. He then served in the U.S. House of Representatives from 1987 to 2007, representing Maryland's Third Congressional District. He was first elected in 1966 to the Maryland House of Delegates while still a University of Maryland law student.
In November, Cardin hosted a town hall in Lanham, where he addressed the latest tax reform bill, among other issues.
"Relief should go to middle-income families," he said in November. "We're losing the middle class in America and they're the ones who need the help."
He also addressed gun safety during the town hall.
"I don't know what more we can do before Congress takes action on gun safety legislation," Cardin said. "We see tragedies every week."
He proposed outlawing high-capacity magazines and military-style weapons, as well as getting rid of law that makes gun licenses valid in all states, not just their states of origin.
In 2012, Cardin won a nine-way primary election with nearly three-quarters of the vote and a four-way general election with 56 percent of the vote, according to the Sun.
Cardin, a vocal Trump critic, said the president's influence on Washington, D.C., will be a central tenet of his campaign.
"I don't believe there will be any question as to where I stand regarding President Trump," he told the Sun.