President Carter and Ronald Reagan were rated the front-runners tonight in a survey of Connecticut voters that showed large numbers are undecided on how they will vote in Tuesday's presidential primary.

According to the survey, Carter has a 63-to-32 percent lead over Sen. Edward M. Kennedy among Democrats with a definite choice. Three percent chose California Gov. Edmund G. (Jerry) Brown Jr., who has not campaigned here.

Among Republicans with a definite preference, Reagan had 43 percent compared to George Bush's 30 percent and Illinois' Rep. John B. Anderson's 22 percent.

But 45 percent of the Democrats and 43 percent of the Republicans interviewed said they were undecided. The phone survey of 1,032 registered Democrats and Republicans was conducted between March 14 and 20, with most of the interviews occurring after last Tuesday's Illinois primary victories by Carter and Reagan.

Weighing the undecideds, Carter has a bit less than 35 percent of all the Democrats and Reagan a bit less than 25 percent of all the Republicans -- indicating the outcome and margins may be very much in doubt.

The survey was conducted by WFSB Television in Hartford by the University of Connecticut Political Science Department.