Muriel Humphrey Brown, who grew from a shy girl from South Dakota into a confident woman during political campaigns for her first husband, Hubert H. Humphrey, and then was named to fill his seat in the Senate at his death in 1978, died Sunday at Abbott Northwestern Hospital in Minneapolis. She was 86 and lived in Plymouth, Minn.

The office of her son, Hubert Humphrey III, the Minnesota attorney general, said only that she died of natural causes after having been admitted to the hospital earlier in the day.In recent years, Mrs. Brown appeared rarely in public, but on Tuesday she stood at the side of her son, Hubert Humphrey III, Minnesota's attorney general, when he won the Democratic-Farmer-Labor Party nomination for governor.

"Hubert would have been proud," she said after her son's victory.

There were many victories in the lives of Hubert and Muriel Humphrey and many setbacks as well. After Humphrey served as vice president under Lyndon B. Johnson, he lost to Richard Nixon in 1968 when Johnson decided against seeking re-election.

Humphrey had also been defeated for his party's presidential nomination in 1960 by John F. Kennedy. Muriel Humphrey was at her husband's side for these elections and when he died after returning to the Senate, she was a logical and sentimental choice to succeed him until the 1978 election, when she did not run to keep the seat.

Because of her shyness she had to work at the roles that came to her.

"If someone had told me what would be happening to me today," she said after the Democrats nominated Humphrey for president in 1968, "well, it would have been an awful thing to have thought of. I just wanted to be a wife."