The new Pentagon budget provides the hardest evidence to date that President Carter envisions a lesser role for aircraft carriers in the future, according to Navy carrier admirals.

They base that contention on the Carter administrations's refusal during recent budget arguments to buy the long-range fighter and attack carrier aircraft they say are needed to combat Soviet weaponry.

Also, sources said, Defense Secretary Harold Brown stressed in budget confrontations with Navy leaders that they should tailor future carriers and their planes to small, non-nuclear wars rather than "power projection" to impress the Soviets.

Carrier admirals, in an argument expected to break out into the open when Congress returns later this month, insist that the administration's fiscal 1977 budget sets the wrong course for naval aviation by the planes and ships chosen.

Brown, over the protests of Navy leaders, stuck with the lightweight F. 18 fighter instead of canceling that program and using the money to buy the more lethal and expensive F-14 fighter and the A-7 attack plane Carter has called obsolete.

A carrier armed only with the F-18 would be good only for "brush fire" wars, Navy leaders complain. Even in a brush fire war, the admirals are complaining, the short-ranged F-18 would force the carrier to sail much closer to shore, making it more vulnerable to land-based defenses.

Brown, in rejecting such Navy arguments, said buying the F-18 at the expense of the F-14 and A-7 would save money in the long run, partly because using the same basic F-18 for both the fighter and attack rules would reduce maintenance costs.

Political fog has settled over the whole F-18 question. House Speaker Thomas P. (Tip) O'Neil Jr. (D-Mass) has become a leading protector of the F-18, which will be powered by General Electric engines built in his home state.

Carrier admirals are also dispirited over Brown's refusal to back off from last year's decision to switch from giant nuclear-powered carriers to medium sized ones.

Currently, the Navy has three nuclear powered aircraft in service - the Enterprise, Nimitz and Eisenhower - and a fourth, the Vinson, under construction. The Vinsion will be the last of the giants if Brown's decision sticks.

Instead of building more giant nuclear carriers, the Carter administration intends to switch to the "midi" and develope a new kind of plane to go with it - V/STOL (vertical and short take-off and landing) aircraft that require only a short stretch of deck for take-off and landing.

Administration officials maintain that this will enable the Navy to buy more carriers, since the midi will be cheaper than the nuclear giants costing about $2 billion each.

Although Navy Secretary W. Graham Claytor has enthusiastically backed the switch to the midi and V/STOL aircraft, he differed with Brown by pushing for the purchase of more F-14 and A-7 planes to give carriers a bigger punch.

Brown approved money for only 24 F-14 rather than the 60 the Navy had hoped for and the 36 it formally requested toward the end of budget negotiations.

Also, Brown refused to approve any money for buying additional A-7 fighters, but went along with the Navy's request for 12 A-6 attack aircraft in a fiscal 1979 budget expected to total about $126 billion.

The new budget amounts to a swing toward small carriers with short-range planes, according to the carrier admirals, and thus threatens to relegate future Navy carrier task forces to brush fire wars and protecting cargo ships.