A member of the Democratic Party, he previously represented the district in the Maryland House of Delegates from 1987 to 1997, and served as the United States Ambassador to Romania from 1998 to 2001.

During his internship, he was the subject of multiple White House memos that questioned whether he should have a voice in the Youth Conference, citing his activism and anti-Nixon views.

Despite these memos, White House aide Steve Hess, who hired Rosapepe, said that he would not consider discharging him as the council also included members of the right-wing Young Americans for Freedom organization, and that having "some radical liberals" in the Youth Conference gave it credibility.

[1] The Baltimore Sun described Rosapepe as a suburban liberal—supporting organized labor, environmental advocacy, and increased education spending—and as an ally of Governor Parris Glendening.

[10] He also became involved with foreign policy issues during his tenure, attending an exchange trip to China in 1982 and extensively traveled to parts of the former Soviet Union.

[22] In 2006, after Governor Bob Ehrlich did not reappoint Rosapepe to the University System Board of Regents,[23] Rosapepe launched a primary challenge against state Senator John A. Giannetti Jr., who was perceived by other Democratic lawmakers as being too conservative after siding with Republicans and Governor Bob Ehrlich on key votes, including a bill to ban assault weapons in Maryland.

[24][25] During the Democratic primary, Giannetti gained national attention after performing the Heimlich maneuver on Rosapepe, who was choking on a piece of lobster at an Italian restaurant they were both eating at.

[1] In January 2012, Rosapepe expressed interest in running for comptroller of Maryland in 2014 if Peter Franchot ran for governor instead of seeking re-election.

[34] He also organized groups of ethnic leaders from Central and Eastern Europe, including Wesley Clark, to support and campaign for Biden in swing states.

[36] During the 1997 legislative session, he led an unsuccessful revolt against House leadership, in which he rallied for increased funding for education in underserved communities and Baltimore.

[44] During the 2008 legislative session, he introduced a bill that would require utility companies that build power plants in Maryland to first sell their electricity in-state, which passed and became law.

[47][48] Rosapepe expressed disappointment with the Maryland Public Service Commission's decision not to place stiff fines against the utility companies in its assessment following the derecho.

[49] During the 1987 legislative session, Rosapepe voted to overturn a Maryland Court of Appeals ruling that allowed victims of crimes committed with Saturday night special handguns to sue the weapon's manufacturers.

[53] In April 2020, amid the COVID-19 pandemic, Rosapepe urged Governor Larry Hogan to form a regional pact with bordering states.

[54] He criticized the Maryland Department of Health's rollout of the COVID-19 vaccine, accusing the agency of creating "artificial shortages" and not providing adequate constituent services.

[59] During his tenure as the president of the Arlington Youth Council, Rosapepe opposed anti-loitering laws that were aimed at teenagers and campaigned for more county funding for educational research.

[73] During the 1991 legislative session, Rosapepe introduced a resolution urging Congress to fund the design and development of a maglev train route connecting Washington, D.C. to Baltimore.

[62] In November 2009, Rosapepe and his wife published Dracula is Dead, a travel literature book that explores the political and social history of Romania during the 20th century.