When administration and congressional leaders sit down Monday to begin negotiations on a deficit-reduction package that may include new taxes, the greatest obstacle to an agreement may be the distrust that has built up over the past seven years.

Despite pledges at both ends of Pennsylvania Avenue this week that the upheavals on Wall Street had raised the stakes too high to engage in partisan rhetoric, neither President Reagan nor the Democrats who control Congress avoided an almost immediate return to form.

In his opening statement at his Thursday night news conference, Reagan said, "When we faced challenges before, this country has resolved them by pulling together." But the next morning, he told a Labor Department audience that "Congress is unable to get control of deficit spending . . . . Congress has made no effort to restrain domestic spending."

On Capitol Hill, House Speaker Jim Wright (D-Tex.) had told reporters on Wednesday, as the prospects for a budget summit were improving, that "it's too late for game-playing and posturing and little meetings and press releases."

On Friday, the press releases were back. Wright issued a statement challenging Reagan's claim that the soaring federal deficits are caused by congressional profligacy, tying them instead to the administration's huge appetite for defense spending and its tax cuts for the wealthy. "The great unanswered question is whether that fundamental truth . . . is yet clear to the president," Wright said.

In spite of Friday's fireworks, there is a widespread feeling in Congress that the financial tremors that shocked the markets have altered the political climate enough to produce a budget deal that has seemed beyond reach all year. But the suspicion between the White House and the congressional Democrats could complicate efforts to get both sides to agree.

"The {market} volatility is based on a feeling that there isn't anyone in charge in Washington," said Rep. Vic Fazio (D-Calif.), a member of the House Budget Committee. "This is a time for statesmanship."

Democratic lawmakers, who have been pushing for a budget compromise all year and for a deficit-reduction package that includes a $12 billion tax increase despite Reagan's earlier opposition, now think that the market has managed to accomplish what their entreaties and bluster could not.

"There's an obvious change," said Senate Finance Committee Chairman Lloyd Bentsen (D-Tex.), who has just shepherded a tax bill through his panel. "He {Reagan} was absolutely stiffing us, and now he's agreed to negotiate . . . . What happened on the stock market had a dramatic effect."

But Reagan was not the only one affected by the markets' turmoil. Democrats have become more willing to discuss further spending cuts.

"I've always felt that Congress was ready to do the difficult things, even more domestic cuts," Fazio said. "All those things are doable if you have an agreement that we are all going to go out and bear the brunt of the criticism."

As Sen. Timothy E. Wirth (D-Colo.) said, the most important political side effect of the financial crisis may be that it creates a situation in which "everybody gives everybody else cover."

If the goal of the talks is the $23 billion in deficit reduction mandated by the revised Gramm-Rudman-Hollings law signed by Reagan last month, not much cover may be needed. Congress, using a balance of half taxes and half spending reductions, was on track for that target already, after already backing off the larger goal of $37 billion called for in its budget resolution adopted in late June.

"It isn't really all that big a deal to get to $23 billion if we take some meaningful expenditure reductions," said House Minority Leader Robert H. Michel (R-Ill.). "The bigger thing for us to meet is the psychology of coming together. That is much more significant, for God's sake, than a relatively few dollars."

The key question is whether $23 billion is enough to restore confidence on Wall Street.

Some lawmakers say no and suggest that Congress use this opportunity to make a far more substantial dent in the deficit that now hovers near $150 billion a year and is projected to creep upward next year. They include Wright, House Majority Whip Tony Coelho (D-Calif.) and Rep. Dick Cheney (R-Wyo.), the third-ranking Republican in the House.

"Let's talk about something that's real," urged Coelho this week, suggesting a figure in the $40 billion to $50 billion range.

That kind of talk, however, gives lawmakers on the two tax-writing committees indigestion.

"The speaker doesn't have the hamburger in his stomach that I do from getting to $12 billion" in taxes, said House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.).

"You don't want to overdo it," added Bentsen.

With both Rostenkowski and Bentsen on the Democratic team that will be negotiating with the administration, the prospect of surpassing $23 billion seems unlikely.

The makeup of the Democratic team also sends a strong conciliatory message to the White House. Leading the House contingent is Majority Leader Thomas S. Foley (D-Wash.), a skilled lawmaker with a nonpartisan temperament. On the Senate side, along with Bentsen and Majority Leader Robert C. Byrd (D-W.Va.) are two moderate southern Democrats, Budget Committee Chairman Lawton Chiles (Fla.) and Sen. J. Bennett Johnston (La.).

But any package that emerges must still be sold to the full House and Senate. Not only are Democrats now in control of both houses, but they are increasingly undaunted by the prospect of challenging a weakened president nearing the end of his term, particularly on an issue on which they believe they hold the high ground.

As Rep. Silvio O. Conte (R-Mass.) said of the upcoming discussions: "I don't think they can afford to fail, but you have some pretty stubborn people here."