The 101st Congress convenes Tuesday and is braced for a quick and politically perilious showdown on pay raises that could spiral salaries of senators and House members to well over $100,000 a year.

The members of the new Congress, trapped in an era of tight budgets and cuts in numerous programs, are terrified of the political fallout from accepting a salary increase of 50 percent.Yet, President Reagan is expected to propose a pay raise, perhaps jumping salaries to as high as $135,000 and, in one way or another, Congress probably will take the gamble and the money.

Congress will meet for two days this week and then abandon the Capitol again until Jan. 20 for the inauguration of George Bush as the 41st president of the United States.

The opening sessions Tuesday, heavy on pomp and tradition, will be restricted to the swearing-in of all House members and 33 senators. The 67 other senators are in the midst of six-year terms.

The following day, the Senate and House will meet in joint session to count the electoral votes from the 50 states, the final and official step in the election of American presidents.

The 101st Congress remains tightly in control of the Democrats, slightly more so as a result of the November elections, setting up the potential for confrontation with Bush similiar to the tension that marked the final two years of Reagan's administration.

But Congress, accepting Bush's conciliatory approach following his highly negative presidential campaign, appears ready to extend the new president a traditional honeymoon. No one is willing to predict how long it will last.

But Democratic leaders have indicated they will give Bush ample time to formulate and propose his agenda - most specifically on how to deal with the budget deficits - and not push their own programs from the beginning of the session.

The Democrats on Capitol Hill will be led by veteran Speaker Jim Wright of Texas and veteran party leader Tom Foley of Washington. But Senate Democrats have their first new leader in a dozen years, second-term George Mitchell of Maine.

Mitchell said that, at the request of Bush, some of the Senate committees will hold confirmation hearings for the president-elect's cabinet members.

"Vice President Bush asked me to do that when we met," Mitchell said but added that actual Senate confirmation votes, sometimes taken on inauguration afternoon, will not come until the Jan. 25th at the "earliest" and was up to the committees.