Sen Gary Hart (D-Colo.). opened a new phase of the great aircraft carrier battle yesterday by urging his colleagues to authorize a big new conventionally powered carrier when they rewrite the defense bill vetoed by President Carter.

"After the smoke clears," Hart said in an interview, everybody will see that we're going to build another big conventional carrier." He noted that the president promised in his veto message to include one in next year's defense budget.

The only real questions, he said, are when it should be added to the defense budget and what kind it should be. He said the Navy already has built one such carrier, the John F. Kennedy, and could build another without piling up the kind of cost overruns that would come from designing and building the 60,000-ton medium-size carrier Carter has in mind for next year.

The Navy, Hart said, would rather have another Kenedy-class carrier than Carter's "midi," now that it seems to have lost its chance of getting a fourth nuclear-Powered Nimitz. The senator said he intends to sound out Defense Secretary Harold Brown on the idea at next week's hearing by the Senate Armed Services Committee, tentatively scheduled for Tuesday.

Hart assailed Carter's veto of the defense bill on the grounds that building one more Nimitz would have ended the carrier battle and enabled the Navy to switch to light carriers of about 25,000 tons. Hart had added money to the vetoed bill to get the Navy started on light carriers.

"Since the Nimitz is not politically possible" now that Carter's veto has been sustained, Hart wrote Chairman John C. Stennis (D-Miss.) and other senators yesterday, and the "midi" carrier "is unjustifiable" because it is "likely" to cost as much as the Nimitz but do a lot less, "the most suitable compromise" is to build another Kennedy.

The Kennedy is almost as big as the Nimitz, Hart said, at 87,000 tons compared to 91.400 tons, and carries about as many aircraft. The Carter administration estimates a Kennedy-class ship would cost $1.7 billion, compared to the $1.575 billion shaky estimate for the new midi and $1.93 billion for the vetoed Nimitz, Hart said.

Hart is also hopeful that by authorizing a large carrier, even though it would be conventional rather than nuclear, the Senate Armed Services Committee could dissuade its House counterpart from deleting the light carrier money that Hart had included in the vetoed bill.