President Carter urged Congress yesterday "to act without delay" on a $24.3-billion-a-year national health insurance plan to protest "all our people" against "devastating" health bills.

Adding more than $3 billion to his aides' recent estimates of the increased federal costs, he sought a massive $18.2 billion in new federal spending, which he calculated in "1980 dollars" though the plan would not being until mid-1983.

With this fund, he said, he woudl expand and combine Medicare and Medicaid as part of a new federal insurance system called "Healthcare," with the overall aim of helping the aged, poor, near-poor, disabled, mothers and infants and many others left uncovered by present health plans.

The president also asked Congress to order employers to contribute an added $6.1 billion so that no employe or employe's family would have to pay more than $2,500 in any one year for medical bills.

Together, Carter said, all these sums would buy protection against catastrophically expensive illnesses for everyone - and do much more in a "first" step toward a "universal, comprehensive" plan to come later.

Sen. Edward M. Kennedy (D-Mass.) promptly charged that the president's plan would entrench and only worsen today's separate, second-class care for the poor - and might also "bankrupt the nation" because of failure to clamp "tough, tight" controls on doctors and hospitals.

Other critics - notably Douglas A. Fraser, president of the United Auto Workers - said "medical costs would go through the roof" because Carter wants to build national health insurance "on a catastrophic insurance base." This, said Fraser, inevitably means paying large sums to doctors and hospitals for the most costly kind of care while still ignoring many acute needs.

In addition, some critics said that, more than two years after President Carter promised a bill in "early" 1978 to establish a universal and comprehensive health insurance system "during my time in office" he was now asking less.

Carter was nonetheless flanked during his presentation by several prominent congressional Democrats, including Sen. Russell B. Long (D-La.). Long heads the Senate Finance Comittee, which must clear any health insurance bill. He has also sponsored a bill to add a smaller amount of catastrophic expense coverage to the nation's health insurance.

Long did not promise to support Carter's plan, but said "all these things are negotiable." Long said he hopes to see a new national health plan enacted to take effect "in 1980," which might mean fiscal 1981 or at least two years earlier than the president's fiscal 1983 target.

Another important Democrat, Sen. Abraham A. Ribicoff (D-Conn.), said he would seek agreement between Kennedy, Carter and Long, since there are many "significant similarities" in their plans. Ribicoff said "we have an opportunity" to pass a bill in this session if everyone "works together."

A main difference between the Carter plan and the broader Kennedy plan - one to add $28.6 billion in federal funds and $11.4 billion in private funds - is that Kennedy and his labor and other backers want to adopt the whole plan all at once, though its parts might take effect step by step.

Kennedy would also negotiate fees and total health spending with doctors and hospitals according to national, state and area budgets set in advance every year. This is a potentially tighter set of controls than those Carter outlined. The president proposes a limit on hospital spending according to a national target, movement toward advance budgeting for hospitals and a set fee schedule for federally paid doctors.

The main difference between the Carter plan and the most recent Long plan is that Carter would offer new coverage for the poor, near-poor, aged and others, in addition to the catastrophic insurance for the employed.

Still, payments for catastrophic illnesses could consume at least half of the president's plan's overall costs, his estimates seemed to show, though many were not broken down by types of illness.

The most likely Senate scenario in this Congress, many poltical observers now feel, is a compromise between Carter and Long, with Kennedy seeking to broaden the final bill.

Asked how he would vote on the Carter plan if - as no one expects - it reaches the Senate floor without being whittled down by Sen. Long. Kennedy said only, "If I had to vote today, I'd vote 'no.'"

What the president asked was a plan to:

Spend $6.1 billion in employer contributions - and another $1.6 billion in federal subsidies for small businesses and low-income workers - mainly to put a $2,500 annual limit on the out-of-pocket medical costs faced by 56 million American workers and their families. Employers would have to cover part-timers who work at least 25 hours weekly for 10 consecutive weeks, and dismissed employes for 90 days. Employes might have to pay another $1.7 billion in premiums, but this would be offset by their savings, Carter said.

Spend $10.7 billion in federal funds to give more nearly equal coverage to 15.7 million people now getting state-run Medicaid for the poor, and extend Medicaid to 14.5 million more low-income persons. Any family spending so much on health care that it sinks drastically under the poverty level would become eligible for a year's federal "Healthcare" coverage.

Allocate $3.9 billion in federal funds to set a $1,250-a-year limit on out-of-pocket costs for 24 million aged and disabled people covered by Medicare, remove present limits on covered hospital days and expand coverage for 5.2 million low-income aged and disabled.

Earmark $300 million in federal funds to offer "Healthcare" catastrophic illness coverage to many persons, young and old, who do not qualify for any current plan and have trouble buying adequate private coverage.

Provide $200 million in federal funds to give full coverage for prenatal, delivery and medical care to 3 million new mothers a year and their babies during their first year of life.

Spend $2.1 billion in federal funds for administration.

These sums add up to $24.9 billion, but the administration said changes in tax flows - for example, fewer personal income tax deductions for medical payments - would give the government $600 million a year in added revenue.

Kennedy said administration officials had told him that the total Carter health plan - when more steps are voted by future Congresses to make it complete - would cost $60 billion in added health spending. But both Carter and Kennedy said their cost controls would mean that before long an expanded health plan would cost the country less than its present health coverage, without controls. CAPTION: Picture, Kennedy on Carter's health plan: "If I had to vote today, I'd vote 'no.'" By James K. W. Atherton - The Washington Post