Capitol Hill copying machines have been busy reproducing mass quantities of a Jimmy Carter letter that urges Congress to be tough, courageous and fair and vote a 7 percent pay raise for federal executives.

"I want to have a copy of it next to my heart," a senior Democrat said, "if the voters put me up against a firing squad for what we are about to do." What he and colleagues are about to do -- and this will be the third time -- is to vote Tuesday on the federal-congressional-legislative pay issue.

Because Congress refused last October's 5.5 percent general federal pay raise, it faces the prospect of two increases this Oct. 1. While rank-and-file government and military personnel get 7 percent, members of Congress and long-frozen federal executives would get a total of 12.9 percent.

To forestall the political repercussions of a 12.9 percent pay raise, members of Congress (who now earn $57,500 in straight salary) are considering several less controversial alternatives:

The same 7 percent raise other federal workers and the military will get this October.

Taking a modified 5.5 percent increase, the one they refused in 1978.

Refusing all pay raises this year.

The latter option is what has President Carter worried. He was worried enough to sign and send a letter to House Speaker Thomas P. O'Neill, asking that Congress play fair with top career federal officials whose $47,500 pay has been frozen since March 1977. Before that, it had been 7 years since the executives had a raise.

The blame for the long executive pay freeze, Carter wrote O'Neill, rests with both Congress and the White House. Some of the points in the Carter letter:

"Many of the career executives are eligible for retirement and will find it economically very advantageous to retire if salaries remain frozen while retirement incomes (which are indexed to the Consumer Price Index) increase rapidly. We may face a serious drain of our ablest and most experienced executives."

Carter said the government's new Senior Executive Service may suffer if it can't offer supergrade-level personnel higher pay in return for trading in job security.

With the rank-and-file 7 percent raise next month, Carter said, more than 9,000 subordinates will move up to the $47,500 pay level of their bosses.

Carter's letter may give courage to some members who want and need a raise, but have lost their nerve in earlier votes. Privately, administration officials have urged members of Congress to allow federal executives a raise, even if they reject any pay boost for themselves. That is unrealistic.

Most members of Congress think they are worth a lot more than the $10,000 difference between themselves and career bureaucrats who don't have to run for reelection every two years. Congress may or may not deserve the raise (it has lots of hidden salary extras and tax breaks). But federal executives can forget about any increases for themselves unless Congress gets the same thing. Odds now are that the Tuesday vote will produce either a 5.5 percent, or nothing.