SANTA BARBARA, CALIF., NOV. 27 -- President Reagan stepped up his campaign for approval of a compromise deficit-reduction plan today by urging Congress to set aside fiscal differences to "preserve our economic gains and keep our nation strong."

Reagan in the letter to Congress said the plan to cut $76 billion from the deficit over two years "maintains vital government services, upholds our national security and preserves the integrity and fairness embodied in last year's tax-reform legislation."

"In an agreement of this magnitude that results from tough bargaining, it is not difficult to find something with which to disagree," Reagan wrote. "Nevertheless, the agreement is our best hope for achieving deficit reduction without resorting to indiscriminate across-the-board cuts that would devastate essential services."

The agreement reached Nov. 20, just before a deadline that triggered possible automatic spending cuts under the Gramm-Rudman-Hollings budget law, would cut $30 billion from the projected deficit for fiscal 1988 through $9 billion in higher taxes, $2 billion in other revenue increases, $11.6 billion in spending reductions and $7.6 billion in asset sales and other savings.

The strongest opposition to the plan has come from conservative Republicans angered that Reagan retreated from his pledge to oppose a tax increase to reduce the deficit and from some liberals who wanted larger cuts in defense spending.