Each Tuesday and Thursday here, the prime minister must appear on the floor of the House of Commons and spend 15 minutes answering questions. At today's session, both questions and answers were crisp.

"Before the prime minister leaves today for Washington," a Labor member from Wales asked in the traditional third person, "will she reflect on the fact that the majority of my constituency hopes that she never comes back?"

Have no fear, Margaret Thatcher retorted. The prime minister will be back.

Thatcher then helicoptered to the airport and took off for Washington, where she was due tonight to deliver the keynote address to a meeting of the International Democratic Union, an organization of conservative political parties. Scheduling and the state of his recovery permitting, Thatcher also will meet with President Reagan.

Thatcher is due to return on Saturday. But an increasing number of her own Conservative Party colleagues appear to wish she would take an extended vacation to reflect on the low state of her popularity -- and by extension theirs -- during the three-month parliamentary recess that begins this weekend.

As one Conservative member of Parliament put it to Thatcher today, "Will you please instruct your ministers -- and perhaps give an example to yourself -- to listen with much greater care in the future to the views of your own supporters or you won't have so many of them."

Six months ago, such public impertinence from the well-regimented Conservative majority would have been unthinkable. The fact that it is now relatively frequent is a sign of the deep trough in which Thatcher finds herself halfway through her second term.

The extent of current disaffection with her tight economic and social policies was apparent in a parliamentary vote Wednesday in which the government -- which holds a 140-vote edge in the Commons -- squeaked by with just 17 votes to spare. Nearly 50 Conservatives voted against Thatcher, and another 50 abstained.

The issue was Thatcher's approval early this week of pay raises of from 12 to 46 percent for top civil servants -- a move viewed as massively insensitive in light of her frequent call for cost-cutting and current pay battles with nurses and teachers, to whom the government has offered 6 percent.

Reports following the 2 a.m. vote described Thatcher as "shaken and white-faced." A senior government official acknowledged that the tally was a "surprise and a disappointment," and that defections far exceeded the expected 20 or so.

Numerous reports from Commons members and in the press said Thatcher barely held the day by implying that she might resign and hold early elections if she did not prevail. When asked today, she did not deny the reports, but said only, "I have to disappoint you. I am here and will remain here."

In public, and to some extent in private, officials close to Thatcher say she has no intention of changing her policies or her way of doing things. They tend to dismiss the disaffected Conservatives as weak-kneed panderers to trendy issues, who do not have the stomach to stick with the Thatcherite revolution.

This week's vote, one said, is "exactly what you would expect to happen at midterm, two years from the next election, in July, on a very touchy subject."

Many political commentators allow some merit to that argument, saying it is far too early to write Thatcher off for a third term.

Yet poll after poll in recent months has shown the prime minister and her party running behind Labor and often behind the third force Liberal-Social Democratic alliance. Each day brings a depressing new statistic of unemployment and a hitch in Britain's economic recovery. Economists across a wide political spectrum plead for the government to loosen its strict hold on public spending to get the country moving.

Friendly voices urge Thatcher at least to give the impression of caring about rising undercurrents of discontent, even if she does not believe them justified. Sympathetic newspapers and magazines run lengthy laments over her seeming obliviousness to the unraveling of her impressive 1983 electoral mandate.

"Ministers no longer hear what is being said in the High Street, in the saloon bar, on the golf course," The Times said today. "They are cocooned in those paneled rooms and limousines, but where, oh where, is their leadership?"