President-elect George Bush assured incoming Senate Democratic leader George Mitchell Thursday he agrees with those pushing the new administration to move first on a deficit reduction plan, but the two men avoided the most contentious budget subjects of taxes and defense spending.

In their first meeting since Bush was elected president and Mitchell, of Maine, was chosen Senate majority leader, the two met over breakfast in the vice president's office. They emerged saying they expect to be able to work together despite anticipated rough spots in the relationship."I heard Senator Mitchell say very clearly (that I) should take the lead on the budget deficit question," Bush told reporters after the discussion. "I assured Senator Mitchell that is exactly what I intend to do."

House Speaker Jim Wright, D-Texas, also has said Bush must take the lead on the budget, and Congress will not begin negotiations until his administration moves first.

Bush agreed Thursday it is his duty to take the initiative but stressed he has not yet put together his entire economic team and is not sure whether his proposal will be new or merely an amended version of President Reagan's plan.

Both Bush and Mitchell reported their meeting was cordial but conceded they avoided discussing some of the most troublesome questions, such as whether to raise taxes and how to treat defense spending.

Bush insisted during the campaign he would fight any tax increases. Yet many congressional Democrats have said they believe new taxes are needed although they are not ready to push the idea without backing from the White House.

Mitchell refused to comment on budget specifics Thursday, saying only that he ruled out any options would make reducing the deficit more difficult.

"I think we all owe to the president-elect the simple courtesy . . . to wait until he has a chance to get his team together and make his plan," Mitchell said. "No purpose is served by our speculating on what he might or might not do and then speculating on what we might then do or not do."

The meeting with Mitchell was one of several Bush has held or plans to hold with political adversaries. He has huddled with Wright, held an amiable session Wednesday with civil rights leader Jesse Jackson and plans to meet Friday with his defeated Democratic rival, Massachusetts Gov. Michael Dukakis.

Despite that cordial atmosphere, Bush acknowledged Thursday he expects some difficult times with Congress. Mitchell added, "We both recognize that we will from time to time differ, but we hope to do so in a cooperative spirit."