Sen. John H. Chafee (R-R.I.), 77, a gentle but stubborn champion of moderation in an increasingly polarized Senate and one of Congress's leading environmentalists, died Sunday of heart failure.

Chafee, a veteran of Guadalcanal in World War II who went on to more than 40 years in politics and government, died at 7 p.m. at Bethesda Naval Hospital after being admitted several hours earlier, according to aides. He had fallen ill at his McLean home. [Obituary on Page B5]

Chafee announced in March he would not seek a fifth term in the Senate next year and never seemed to recover full strength after undergoing back surgery during the summer. But he maintained a full schedule and was honored recently for a variety of causes, including last Thursday night for his support of historical preservation.

President Clinton said Chafee "embodied the decent center which has carried America from triumph to triumph for over 200 years" and "proved that politics can be an honorable profession."

The impact of Chafee's death will probably be felt most immediately on the Senate Environment and Public Works Committee, which he headed for the past five years, giving the panel an environment-friendly focus that was not always in tune with the majority of Senate Republicans.

Conservative Sen. Robert C. Smith (N.H.) would be in line to succeed Chafee but for the fact that he left the Republican Party a few months ago to pursue the presidency as an independent, leaving his claim to the chairmanship in serious doubt. Next in line is Sen. James M. Inhofe (R-Okla.), another staunch conservative with a record that could hardly contrast more dramatically with Chafee's.

During their careers in Congress, the League of Conservation Voters found that Chafee agreed with its position 70 percent of the time, compared with 7 percent for Inhofe and 36 percent for Smith. The choice of a successor rests with the largely conservative Republican members of the committee, subject to approval by all GOP senators.

Chafee's seat as a senator is almost certain to be filled by a Republican, most likely by his son. Under Rhode Island law, Gov. Lincoln Almond (R) will appoint a successor to fill the remaining 14 months of Chafee's term. Chafee's son Lincoln, mayor of Warwick, the state's second-largest city, is the only active candidate for the Republican nomination for next year's senatorial race. Almond indicated yesterday he will not make a decision for at least a week.

A Yankee blue-blood with an unassuming manner, Chafee was a three-term governor and secretary of the Navy before being elected to the Senate in 1976, repeatedly winning reelection in one of the country's most Democratic states.

At risk of retribution from conservative critics, he supported stronger environmental protections, abortion rights, gun control and expanded health care coverage, especially for women and children. He was one of five Senate Republicans who rejected both articles of impeachment against President Clinton and, more recently, one of four Republicans to vote with Democrats in the failed effort to ratify a treaty banning nuclear weapons tests.

Chafee ranked third in the Senate Republican leadership until conservatives ousted him after the 1990 elections. Some conservatives wanted to deny him chairmanship of the environment committee after Republicans took control of Congress in 1995 but backed off.

As flags were lowered to half-staff above Capitol buildings and a display of violets, carnations and orchids was placed on his desk, colleagues of both parties took the Senate floor to speak of Chafee and his record, often in strikingly personal terms.

"In a city that is known for its partisan rancor, John was a champion of consensus and bipartisanship. I don't know anyone that didn't like and respect John Chafee," said Sen. James M. Jeffords (R-Vt.), a close friend and frequent ally of Chafee's in the dwindling band of Senate GOP moderates.

Senate Majority Leader Trent Lott (R-Miss.) called him a "genially independent New Englander, respected on both sides of the aisle, who worked to bring opposing sides together for a common goal." Minority Leader Thomas A. Daschle (D-S.D.) called him a "public servant in the fullest and finest sense."

Staff writer Linda Wheeler contributed to this report.

CAPTION: John Chafee was a former Marine, governor and secretary of the Navy.