Sentinel, a conservative group that lobbies on foreign affairs, spent $50,000 during the past nine days on television spots that blasted Rep. Michael Barnes (D-Md.) for his opposition to President Reagan's plan to provide federal aid to rebels fighting the government in Nicaragua.

Barnes was one of about a dozen members of Congress targeted by the group in the ads, said Carl Russell Channell, president of Sentinel.

Barnes, chairman of the Foreign Affairs subcommittee on the Western Hemisphere, has been under attack by conservative groups and Republican House members for his leadership role in opposing aid to the counterrevolutionaries, also known as "contras."

Barnes, who is running against Rep. Barbara Mikulski and Gov. Harry Hughes for the Democratic Senate nomination, called the ads "another obvious attempt by the radical right to influence the outcome of the U.S. Senate race in Maryland."

Barnes' supporters have made comparisons between his situation and that faced by Sen. Paul Sarbanes (D-Md.), who was targeted by the National Conservative Political Action Committee in 1982.

NCPAC's negative ads became the focus of the campaign and were thought to have contributed to Sarbanes' overwhelming victory.

"Last March, when a related right-wing group and the Maryland Young Republicans launched a similar attack against me, I warned that those attacks were part of a long-term negative campaign to defeat me in Maryland's primary election," Barnes said.

Channell, who also heads two other conservative groups -- the National Endowment for the Preservation of Liberty and the American Conservative Trust -- said the recent ads were aimed at influencing the House debate over contra aid, not the Senate race.

But Channell said he plans an aggressive statewide offensive against Barnes, to begin later this summer.