President Reagan plans to abolish the Community Services Administration -- a vestige of Lyndon Johnson's War on Poverty -- and cut back considerably the controversial anti-smoking program set up in the Carter administration by former Secretary of Health, Education and Welfare Joseph A. Califano Jr., government sources said yesterday.

At the same time, trade ambassador and former GOP national chairman Bill Brock warned congressional Democrats, in remarks at a breakfast, that Reagan will not hesitate to veto appropriation or authorization bills that threaten the grand design of his budget-cutting proposals.

Brock said he expects the president to achieve most of his tax and budget goals without such drastic action, but added, "We have raw power and the ultimate weapon is the veto power." He suggested that Democrats could lose control of the House in 1982 if the public believes they are frustrating Reagan's initiatives.

In still another budget development, White House press secretary James Brady denied "categorically" that Reagan plans to cut Veterans Administration medical programs in a major way. "We're not getting rid of doctors, nurses, existing medical programs, but there will be some marginal adjustments made," Brady said. On Tuesday Rep. Robert Edgar (D-Pa.) created a stir in the Veterans Committee when he said he had learned Reagan was proposing dismissal of 20,000 medical personnel.

The Community Services Administration (CSA) was a successor to the old Office of Economic Opportunity, the original anti-poverty agency. President Nixon sought to abolish OEO and succeeded in having most of the larger anti-poverty programs -- Job Corps, Head Start, Follow Through, VISTA and several others -- moved out to other agencies. CSA was created to house what as left. This year CSA, which makes grants to community organizations in low-income areas for a variety of purposes, has a budget of $541 million.

The administration several weeks ago revealed that it wanted to combine 40 health and social service programs that now consume about $9.3 billion a year into one or several "block grants" to the states, and cut their cost 25 percent. Fuel assistance to the poor was one of these. It turns out CSA is another, meaning CSA as a federal agency would disappear.

The agency appealed this proposal to the Office of Management and Budget, but sources there, at CSA and at the Department of Health and Human Services said that as of yesterday, it appeared the appeal would be turned down. But one source said CSA has been in and out of the block grant proposal several times recently, and the final decision remained unclear. g

The anti-smoking program, with about 24 employes and a $3 million budget, was a pet project of Califano himself an ex-smoker. It became a great irritant in tobacco states, and caused the Carter administration enormous political problems.

In addition to gathering and sending out statistics and information on the health hazards of smoking, it financed anti-smoking ads on television and in other media. Sources said the budget proposals originally wiped it out altogether, but after an appeal, HHS Secretary Richard S. Schweiker (also a nonsmoker, according to reports) agreed to restore some funds.