Howard Baker Jr., who played key roles in Ronald Reagan's presidency as legislative emissary in the Senate during the administration's triumphant first year, and as a steadying hand inside the White House during its troubled later years, has died.

Baker, 88, died Thursday at his home in Huntsville, Tenn., according to John Tuck, a senior adviser in the Washington office of Baker Donelson, the law firm where he was senior counsel. The cause was complications from a stroke he suffered on June 21.

The namesake son of a seven-term congressman and son-in-law of Senate Republican leader Everett Dirksen, Baker represented Tennessee in the Senate for 18 years, rose to majority leader and ran unsuccessfully for president before replacing Donald Regan as President Reagan's chief of staff.

His 17 months at the White House, from March 1987 to July 1988, coincided with a dark time for Reagan. The president's detached management style was under criticism. Democrats had retaken control of the Senate, and Reagan's prostate surgery had raised concerns about his stamina.

Baker was credited with restoring order and purpose to the White House and helping to improve relations with a restive Congress. On Baker's first day, Reagan wrote in his diary: "He's going to be fine & there was a great feeling in the West Wing of improved morale."

Known as a straight shooter who valued civility in politics, Baker had first made his name in Congress as the vice chairman and top-ranking Republican of the Senate committee that investigated Watergate. During the committee's hearings, he articulated the question that became part of the American political lexicon: "What did the president know, and when did he know it?"

He became the Senate's Republican leader during the presidency of Democrat Jimmy Carter, and then majority leader after the 1980 election. Baker called himself Reagan's "spear-carrier."

Baker was known for his gentility and conciliation toward colleagues of both parties, a reputation that generated suspicion among hard-core Republicans when he ran unsuccessfully for president.

Howard Henry Baker Jr. was born on Nov. 15, 1925, in the Cumberland Mountains town of Huntsville, Tenn.

He declared his candidacy for president in 1979 and was viewed as a leading contender for the Republican nomination. After third-place finishes in Iowa and New Hampshire, Baker became the first high-profile candidate to drop out.