President Reagan acknowledged last night that the economy will not recover from the current recession as rapidly as he has long predicted and cautioned that recovery would require "enormous effort and patience" from the American people in the days ahead.

In the opening statement of his 12th formal news conference, Reagan said he did not intend to change his economic strategy or resort to any "quick fixes," declaring, "slowly and surely, we're working our way back to prosperity."

Constructing a defense against the certain volleys of criticism over the recession by Democrats in the fall congressional campaign, Reagan blamed current economic problems on "back-to-back decades of red ink spending," mostly by Democratic administrations and Democratic-controlled Congresses, and the failure of the current Congress to approve all the tax and spending cuts he sought.

Calling for public pressure on Congress to approve a constitutional amendment that would require a balanced budget, Reagan said that only such a "fundamental reform" would insure that the mistakes of the past would not be repeated.

Reagan said hopeful signs were mixed in with his bleak, new assessment of the short-term economic outlook. Citing a recent drop in short-term interest rates, a decline in inflation over the past year and increases in retail sales, he said, "I think we're going to see an improvement in the second half of this year."

But Reagan emphasized that the task of restoring the economy to health would be "tough, slow work."

"Many of our people are still suffering and nothing has been more painful to me than the slowness of our progress," he said.

Issuing a strong personal appeal for patience, Reagan said, "If we have the courage to believe in ourselves and stop wringing our hands, roll up our sleeves and get the job done, and for once get it done right, we can start repaying that mortgage on our future and create opportunity and hope again for every American."

Immediately after the news conference, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) accused Reagan of "trying to hide from his own record," adding that, "As Joe Louis once said, 'He can run but he can't hide.' "

O'Neill said the record shows that Congress gave Reagan everything he requested last year--his tax program, budget and a reconciliation bill that was drafted by Reagan's own budget director. But, instead of creating 13 million new jobs and balanced budgets, the speaker said, Reagan's economic program has "thrown 3 million more Americans into unemployment" and led to the "highest federal deficits in history."

In recent weeks, senior White House aides have begun to back away from the administration projections for the economy. They fear that if they continue to exude optimism about a rapid turnaround they could jeopardize Reagan's credibility when it is plain that there is no substantial movement out of an economic plight that has resulted in the highest levels of unemployment and bankruptcies since the Great Depression.

White House polls and other surveys indicate that most people still blame the Democrats for the economic problems and are willing to give Reagan's recovery program up to a year to show beneficial effects if they see some signs that it is beginning to work.

In his performance last night, Reagan clearly sought to take maximum advantage of these sentiments.

In his responses, Reagan refused to name a date when prosperity would return or when and by how much interest rates would fall. And he signaled that there might be more bad news before the nation gets "back to a growth economy that will be based on solid principles."

"As I say, we are in that transition period," he said. "There will be some indices, economic indices that will turn up bad, such as the 1 percent monthly increase in inflation, but I don't take that as a permanent switch to double-digit inflation at all. And I think that we're going to see an improvement in the second half of this year."

"I am not going to try to project exactly what level it will reach and exactly what date it will reach that level," he said. "I don't think anyone can."

But he made it clear that he has no intention of retreating from his policies. Although he blamed high deficits for current problems, he held open the possibility of raising his defense budget in future years. He said he would find corresponding spending cuts elsewhere.

Reagan made it clear that Congress and what he often describes as its penchant for "runaway spending" would continue to be one of his targets. Suggesting that much of the barrier to recovery may be "psychological," Reagan suggested that bankers and financial markets still fear Congress would not exhibit "fiscal integrity and common sense."

It was for that reason, he said, that he was "trying to get a little more publicity for the American people to urge their congressmen to adopt the balanced budget constitutional amendment."

One questioner asked if he didn't think that, given the record deficits in his budget, his campaign for balanced budgets was ironical because he was "telling the American people, in effect, there ought to be a law against what I am doing." Reagan bristled.

"The budget deficits I don't think can be laid at an individual's door," he said. "We have--I could turn around and say how much less that deficit would be if the Democratic leadership that is now pointing this nice little thrust that you just repeated--if they had given us all that we asked for last year and this in reductions in government spending--but we have never gotten yet what we have asked for."

"I don't feel self-conscious at all," Reagan said.