PRESSURE COOKER. Lots of tough politicking left for last weeks of Congress's term

Members of Congress have pumped thousands of hands, eaten scores of hot dogs and watermelon at hundreds of picnics, and paraded down Main Streets across America in Fourth of July festivities. Today, they leave all that to return to Washington and confront the prospect of a steamy summer's worth of high-intensity politicking on Capitol Hill.

During the next five weeks, lawmakers will pick their way through the budget and tax mine fields. They may also handle an assortment of other political explosives, such as a mammoth bill aimed at reducing the nation's trade deficit, as well as the nomination of Judge Robert H. Bork to the US Supreme Court.

The resulting political temperatures inside the House and Senate chambers could rival the asphalt-buckling climate outside. Though the presidential election is 18 months away, the preliminary jockeying among the candidates and between the parties is the backdrop against which Congress is laboring.

The Democratic majority has asserted itself against President Reagan, eager to establish a legislative track record that will help catapult a Democrat into the White House. Republicans, on the defensive as never before during the Reagan era, are determined to derail the Democratic agenda and have mustered the cohesiveness to thwart a few important Democratic initiatives.

Ronald Reagan, for his part, seems determined not to go quietly into the night. His clout has been diminished as his party looks for a successor, his prestige damaged by the Iran-contra affair. But President Reagan has come back swinging, taking to the road to champion the conservative causes he has espoused throughout his political career and to drum up public support for budget-related items Congress has repeatedly rejected, such as a balanced-budget amendment.

Indeed, this month's great struggle is almost certain to occur over the perennially volatile budget and tax issues. Congress settled on a budget blueprint for fiscal 1988 that calls for $19.3 billion in new taxes, although Mr. Reagan has expressed unyielding opposition to any form of a tax increase. The Democrats passed the budget with virtually no Republican support, and they cannot count on much Republican help as they attempt to implement the $1 trillion plan.

The hostage in this duel of wills is the annual federal budget deficit, which currently exceeds $170 billion and which the Democratic budget would reduce to about $135 billion in the next fiscal year, beginning Oct. 1. The budget plan relies heavily on tax increases, without which next year's deficit would exceed $150 billion.

Democrats are concerned that a deficit of that size might fuel an election-year perception that they are not serious about deficit reduction. At the same time they are wary of being branded as ``tax-and-spend Democrats,'' particularly if President Reagan's antitax rhetoric wins over public opinion.

So they are searching for a tax-increase formula - raising excise taxes on cigarettes and liquor, for example - that will generate public support and pressure the President into signing a tax-raising bill. If that fails, some Democrats contemplate trying to score what political points they can, while they can - for example, by sending the President a bill that would raise income taxes for the very wealthy, something he is sure to veto.

Similarly, key Democrats are supporting a strategy designed to present the President with an unattractive alternative to a tax increase. House and Senate Democrats are trying to design some sort of constitutionally acceptable replacement for the Gramm-Rudman act's automatic budget-cutting mechanism, which was struck down last year by the Supreme Court. Then, if Reagan refused to sign a tax bill, the resulting deficit would trigger automatic spending cuts in military as well as domestic spending.

The strategy promises to put the President in an awkward position. Faced with the prospect of drastic reductions in defense spending, Democrats believe Reagan will either sign the tax bill or override the Gramm-Rudman mechanism, thus embarrassing himself.

Then again, Democrats have learned to be wary of Reagan's political skills. The Gramm-Rudman legislation is likely to be attached to a bill raising the federal debt ceiling which must be passed this month. But the resolution of the matter will not occur until after a three-week recess in August, when Congress will throw together a massive spending bill to keep the federal government in business during the next fiscal year.