Mixed emotions, according to an old joke, is watching your new, uninsured $25,000 Mercedes plunge down the Grand Canyon with your mother-in-law at the wheel.

Politically insecure members of Congress are suffering an attack of mixed emotion jitters these days. Their problem: Unless they do something within the next couple of weeks they will be "forced" by law to give themselves a double-dose pay raise this October worth about 13 percent.

Members of Congress have a very small rooting section for the pay raise. It consists of about 12,000 federal executives, political appointees and judges whose pay rates and fortunes are linked to whatever Congress does about its own compensation.

President Carter has asked Congress to give white collar federal workers and the military a scaled-down pay raise of 7 percent this October.That is more than the 5.5 percent he originally proposed but less than the 10.4 percent the government says he is justified to put federal pay on a level with counterpart industry raises.

Under the federal pay act, members of Congress are supposed to get the same increase as federal and military personnel. That raise this year, assuming Congress does not veto it, will be 7 percent. To compound the problem of senators and representatives, they are also due the 5.5 percent raise they passed up in October 1978 because of its proximity to the November elections.

As things stand now Congress has to do something or it will get the two raises, 5.5 percent compounded and 7 percent. in October.

The vehicle for adjusting congressional pay is the Legislative Branch Appropriations Act. The House killed the Capitol Hill money bill earlier this summer after getting the jitters about its own pay. But it must do something, and soon, to approve one or both of the raises or to specifically exclude itself from any increases.

If it decides -- as some insiders believe it will -- to take the 5.5 percent 1978 raise, pay for federal executives now frozen at $47,500 will rise automatically by about $2,600. If, however, Congress decides this is the wrong time for any pay raise for itself, federal executives will remain frozen at the $47,500 level. Many subordinates who will get the 7 percent federal pay raise will move up into the same salary bracket as their bosses.

White House officials are working quietly with the leadership on Capitol Hill (especially in the House) for a 5.5 percent congressional pay raise. They figure it is the least painful decision Congress can make and still allow restless government executives to move up along with their subordinates.

One congressional expert said he guesses that Congress will wait until the last minute (around the end of September) to quietly approve a 5.5 percent raise for itself and other top federal officials. "But if this thing gets too much publicity, and too much heat from the public," he predicted, "it could kill off raises for this year and 1980 because Congress sure as hell isn't going to vote itself any kind of raise in an election year." He predicted that the next two weeks will determine whether Congress takes a small raise this year, or decides to freeze itself and the upper reaches of the bureaucracy for another two years.