WASHINGTON (AP) _ Noting that one of seven Americans still held hostage in Lebanon is nearing 500 days in captivity, two congressmen vowed to keep the hostage issue before the American public daily.

''From now on, not a day should pass without talking about these people,'' Rep. Robert K. Dornan, R-Calif., said Tuesday.

''The seven Americans ... seem to be slipping from the public view again,'' said Rep. George O'Brien, R-Ill., in a speech on the House floor.

The two congressmen brought up the hostage issue during a period reserved for one-minute speeches on any subject at the start of each House session. Dornan said he wants two House members to speak each day on the House floor about the hostages.

O'Brien said that TV networks and daily newspapers, for the most part, have not run stories in the past week about the seven Americans. A radical Shiite Moslem group known as the Islamic Jihad has claimed responsibility in the kidnappings.

One hostage, political officer William Buckley, is nearing his 500th day of captivity, said O'Brien.

In an interview, Dornan said he thinks the Reagan administration should face the fact that it has a hostage situation that has lasted longer than the Iranian hostage crisis during the Carter administration.

Buckley, he noted, has been in custody longer than the American diplomats seized at the U.S. Embassy in Tehran in November 1979 and held for 444 days.

Besides Buckley, those in custody include: Terry Anderson, chief Middle East correspondent for The Associated Press; the Rev. Benjamin Weir, a Presbyterian minister; the Rev. Lawrence Jenco of the Catholic Relief Services; and three Americans associated with the American University in Beirut, Peter Kilburn, David Jacobsen and Thomas Sutherland.