War on infant mortality, which kills 11 of every 1,000 U.S. babies in their first year of life, was declared yesterday by an unusual political coalition that includes Sens. David F. Durenberger (R-Minn.), Ernest F. Hollings (D-S.C.), Edward M. Kennedy (D-Mass.) and Strom Thurmond (R-S.C.) and Reps. Henry A. Waxman (D-Calif.) and Henry J. Hyde (R-Ill.).

The coalition said it was introducing legislation to allow the states to provide prenatal, delivery and post-delivery Medicaid care to any pregnant woman and any child up to one year of age in families below the poverty line, without adding the family to the cash welfare rolls.

Coalition members were joined by Gov. Richard W. Riley (D-S.C.), head of a governors' task force on infant mortality, at a Capitol Hill news conference.

"The United States ranks 16th among developed nations in infant death rate," Thurmond said, "and South Carolina has one of the highest rates in the nation . . . 15 per 1,000."

In 1985, the U.S. mortality rate was 10.6 per 1,000 live births; in 1983, the 6.8 percent rate of low-birth-weight babies (those under 5.5 pounds) was higher than in a dozen other developed nations.

For 1983, the latest state figures available show infant mortality rates of 19.3 per 1,000 live births in the District, 11.8 in Maryland and 11.9 in Virginia.

In a letter to Senate colleagues, the four senators said there are "3.4 million women of child-bearing age living in households below the federal poverty line who are not covered by Medicaid. These women have little access to prenatal services."

Added Kennedy, "Death rates for children of low birth weight are 40 times higher in the first month" than for children of normal weight.

The coalition said its proposed legislation could substantially reduce the chance of low birth weight and abnormalities by encouraging pregnant women to seek medical treatment of conditions that can cause unhealthy babies, such as poor nutrition.

Durenberger, Sen. John H. Chafee (R-R.I.) and others are expected to push legislation in the Senate Finance Committee, which has Medicaid jurisdiction.

Waxman, announcing that Reps. Mickey Leland (D-Tex.) and Hyde are the "lead sponsors in the House," said the bill is backed by a majority of the House Energy and Commerce subcommittee on environment and health, which handles Medicaid.

In the Senate, the bill's estimated $100 million annual cost has already been accounted for in a budget resolution through the efforts of Hollings and Sen. Lawton Chiles (D-Fla.).

"Studies show that for each percent decline in low birth weight, $24 million is saved," said Hollings. "We save $3 for every dollar invested."

Currently, low-income families with children normally get Medicaid only if they are eligible for cash welfare, but most of the states have an eligibility cutoff substantially below the poverty line, which is $10,600 a year for a family of four. Other states do not provide benefits to families with an unemployed father present no matter how low the family income.