Washington — Rep. Floyd J. Fithian (D) of Indiana will share the Democratic Party ticket in November with President Carter, but his constituents might never guess it. He reminds northwestern Indianans at every opportunity that he opposes the President's Soviet grain embargo, his hike in gasoline taxes, and his phase-out of a curb on steel imports.
"When Mr. Carter is right I don't hesitate to say so, but when he is wrong I don't hesitate either," the three-term congressman is fond of telling voters.
If Mr. Fithian often sounds as if he is running against his fellow Democrat in the White House rather than in tandem with him, such impolitic independence may be the price of political survival.
Mr. Carter's name at the top of the ballot does not loom as much of an electoral asset in Rensselaer, Valparaiso, or the rest of Indiana's second congressional district -- one of the most dependably Republican districts in the country in presidential elections.
His own soundings lead Fithian to worry that Republican presidential nominee Ronald Reagan may carry his district by as sweeping a margin ad did President Nixon in 1972, when Democrat McGovern mustered just 26 percent of the vote.
This congressional election campaign is full of Fithians -- Democratic lawmakers seeking in their re-election races to distance themselves, in various ways and degrees, from a Democratic President whose coattails look exceedingly short.
When he captured the White House in 1976, Carter ran behind 281 of the 292 Democrats elected to the House of Representatives along with him, and 19 of the 20 Democrats elected to the Senate.
Four years later, his ability to assist his congressional compatriots seems little improved.
A survey of roughly one-half of the House districts now represented by Democrats, conducted last month by the respected House Democratic Study Group, finds the President trailing either Mr. Reagan or independent candidate John B. Anderson (or both) in 80 percent of them.
Candidly conceding the liability he imposes on many congressional campaigns, Carter went as far as to tell a group of House Democrats recently that, as reported by one of the participants, he would not campaign in areas where his party's candidates feel his presence would "pull them down."
The go-it-alone strategy of many Capitol Hill Democrats is summed up by one jittery junior representative who is defending a marginal seat in a northern industrial state: "Candidates must be their own man or woman, and speak for themselves."
Here is how some of them are doing so:
* A substantial number of congressional Democrats, including four senators up for re- election, openly criticize attempts by the President and their party to exclude Mr. Anderson from televised presidential debates.
Says one of them, Rep. Peter H. Kostmayer (D) of Pennsylvania, whose swing district in suburban Philadelphia contains more Republicans than Democrats: "Efforts to stifle political competition and involvement can only increase the public's cynicism."
* Sen. John A. Durkin (D) of New Hampshire, a state with high energy costs and an equally high Republican voting tradition, campaigns against the energy program of his Democratic ticket-mate. He blasts the Carter energy policy as "a snow job" that wages economic warfare against New England.
* Sen. Donald W. Stewart (D) of Alabama has called for a federal tax cut just as fervently as the President has opposed one this year.
* Sen. Birch Bayh (D) of Indiana, locked in a tight race for re-election in a state that Carter lost handily in 1976, has accepted the chairmanship of the special Senate subcommittee probing Billy Carter's ties to Libya -- a job unlikely to endear him either to the President or the Carter faithful.
He has ventured the early opinion that the administration may have been guilty of "misjudgments" in the case.
Another Democratic senator campaigning for re-election, Dale L. Bumpers of Arkansas, also has accused the President of "every bad judgment" in the Billy Carter affair.
* A group of 50 to 60 House Democrats rebelled against their party standard-bearer earlier this summer by organizing an unsuccessful drive to "open up" the party's national convention to allow Carter delegates to vote for another nominee.
"We're on the eve of what appears to be an absolute disaster for our party," warned the group's spokesman, Rep. Michael D. Barnes (D) of Maryland, who reckons Carter is running third in his district.
* Sen. George McGovern (D) of South Dakota has made Carter a prime target of his uphill campaign -- criticizing him on draft registration, human rights policy, failure to achieve a strategic arms treaty, the MX missile, and overall "shortage of leadership."