Congress as a whole may be marching in tune with the conservative Reagan era, but there could be hope for the younger generation, according to Americans for Democratic Action, the liberal lobbying organization.

The ADA gave Congress a poor grade generally for its support of liberal programs and policies in 1985 but took hope from younger members, particularly those elected in 1984. The group also noted a steadily widening gap between the support level of Democrats and Republicans for its issues since 1981.

The average support for 20 selected issues, including opposition to the Gramm-Rudman-Hollings budget-balancing law and funding for the MX missile and President Reagan's "Star Wars" missile defense proposal, was 40 percent in the Senate and 45 percent in the House. Freshman senators had a 79 percent support level on these issues, however.

Five senators and 20 representatives, all Democrats, got 100 percent ratings. The senators are Alan Cranston of California, Tom Harkin of Iowa, Gary Hart of Colorado, Howard M. Metzenbaum of Ohio and Paul S. Sarbanes of Maryland.

Sens. Charles McC. Mathias Jr. of Maryland and Lowell P. Weicker Jr. of Connecticut were the highest rated Republicans with "passing grades" of 70 percent. Eighteen GOP senators and 17 representatives got zero ratings.