House Democrats ended a weekend conference here today with a sharp attack on President Reagan's policies and pledges to try reversing what Majority Leader James C. Wright Jr. (Tex.) called "this downward trend" brought on by the Reagan administration.

At a news conference with other House leaders, Wright accused Reagan of ignoring the dangers of the bulging federal deficit to achieve his "primary goal of dismantling the domestic side of the government."

By blaming the deficit on excessive domestic spending, Wright said, "The president is engaging in the greatest and most dangerous example of fantasy that I have ever seen anyone engage in in the White House."

He added, "I'm not sure he will ever face reality."

With that partisan attack, more than 100 Democratic lawmakers at the House Democratic Caucus meeting at the posh Greenbrier resort prepared to return to Washington for Reagan's State of the Union address Tuesday and their first look later this week at his fiscal 1987 budget.

Preparing for an expected brutal budget battle against the backdrop of the new Gramm-Rudman-Hollings balanced-budget law, the Democrats promised to offer "choices for change" during the year but stressed that the Republican administration and GOP-controlled Senate hold most of the power.

"We have responsibility to act and lead, but we don't have control of the outcome," said caucus chairman Richard A. Gephardt (Mo.). "That puts a political party in a very difficult position."

The Democratic leaders made clear that the budget battle is about to begin with no sign of a break in the deadlock with the administration over taxes and spending.

If that deadlock persists through the congressional session, the new Gramm-Rudman-Hollings law is expected to trigger more than $40 billion in automatic spending cuts Oct. 1. As long as Reagan remains opposed to any tax increase, Wright said, "there won't be a tax bill" emerging from Congress this year.

Rep. Tony Coelho (Calif.) added that, even if Reagan agrees to some revenue-raising measures, "this Congress is not going to pass an individual [income] tax increase."

While Democrats generally agree that the federal government needs more revenue, Coelho said they favor higher corporate taxes or an oil-import fee.

Similarily, Majority Whip Thomas S. Foley (Wash.) said later that Social Security benefits, which Congress exempted from the threatened automatic cuts, will remain untouchable in any deficit-reduction compromise. "There is just a consensus against doing anything on Social Security," he said.

Foley and other House leaders said the Democrats may approve some domestic-spending cuts sought by Reagan in the budget he submits to Congress this week. "Personally, I don't think that every domestic program has to be maintained forever," he said.