President Reagan should show his support of the Nicaraguan cease-fire agreement by refraining from pushing for renewed military aid for the Contra rebels, a House Democratic leader said Saturday. ``Old policies _ like old habits _ die hard. We must resist the temptation to say `we know better' by approving military aid before the peace plan has a chance to succeed,'' Rep. Tony Coelho, D-Calif., said in the Democrats' weekly radio address. Coelho, the assistant House majority leader, said: ``The burden on President Reagan is to recognize that diplomacy works ... and make peace his highest priority.'' The president, commenting Friday on the cease-fire accord reached Wednesday, said ``there is reason to have caution'' about whether Nicaragua's Sandinista government will abide by the pact. The Sandinistas ``have a past record that indicates that we should be'' cautious, Reagan told reporters. The administration is pushing for approval of a package of humanitarian aid to the Contras before Congress recesses for Easter. Coelho said House Speaker Jim Wright of Texas is ready to schedule a vote this week. ``If our Republican colleagues help us write and pass a plan, we can speed humanitarian aid to the Contras. And the chance for a truly bipartisan policy toward Central America would be at hand,'' Coelho said. Earlier this month liberal Democrats opposed to any aid for the Contras and conservative Republicans pushing for military aid teamed to defeat a humanitarian aid bill offered by Wright.