Howard Baker: US Senate leader and Chief of Staff whose moderate brand of Republicanism is today a vanishing force

"What did the president know and when did he know it?" Those words – among the most famous lines in all American politics, encapsulating the Watergate scandal – were delivered by Howard Baker, then Senator from Tennessee and throughout his career the standard-bearer of a moderate Republicanism that today is all but extinct.

The very question, which he asked repeatedly as the senior Republican on the Senate Watergate committee headed by North Carolina's Sam Ervin, summed Baker up. Initially he could not believe that Richard Nixon was involved in the criminal shenanigans of his staff. But a parade of witnesses before the committee in 1973 and 1974 gradually convinced Baker otherwise. He was a party loyalist; but for him, partisanship was nothing when compared to the national interest.

His powerful performance during the televised Watergate hearings turned Baker into a national political figure. In fact he might not have been there at all had he accepted an offer from Nixon in 1971 for one of two vacant seats on the Supreme Court. But he dithered, and the president lost patience, nominating William Rehnquist, later chief Justice under presidents Reagan, Clinton and George W Bush.

The Court's loss was the Senate's gain. Baker steadily rose through the ranks, to minority leader during the presidency of Jimmy Carter, and then majority leader when the Republicans recaptured control of the Senate in the watershed 1980 elections that swept Ronald Reagan to the White House.

Politics and public service were in Howard Baker's blood: his grandfather was a judge, his father a seven-term Congressman for an eastern district in Tennessee – and his marriages only reinforced that gene. The first, lasting until her death, was to Joy Dirksen, daughter of Everett Dirksen, a legendary Republican grandee in the Senate of the 1960s. His second, in 1996, was to Nancy Kassenbaum, daughter of Alf Landon, the 1936 Republican presidential nominee against Franklin Roosevelt, and herself a Republican senator.

A first Senate run in Tennessee was unsuccessful. The second time around, in 1966, Baker fared better and became the first Republican to win a state-wide popular election since the Reconstruction era, after the Civil War. He did so thanks to moderate policies that attracted blacks and many traditional Democratic voters. He carried that approach to the national stage in Washington.

Baker was the quintessential mediator, courtly in manner and ever civil, a backstage dealmaker who liked to describe the Senate cloakroom, the private rooms next to the chamber where members of both parties would congregate, as his office. His methods earned him the nickname of "The Great Conciliator". But his gift for compromise would doom his pursuit of national office.

In 1976 Gerald Ford, under pressure from conservatives, passed Baker over as his running mate in favour of the more partisan and sharp-tongued Bob Dole. Four years later he sought the White House himself, but by then the party's right wing was carrying all before it and Baker ended his campaign after defeats in Iowa and New Hampshire. As Senate majority leader he could not have been a more loyal Reagan lieutenant, helping push through the president's sweeping first-term tax cuts. But in 1984 he announced his retirement, ostensibly to spend more time with his cancer-stricken wife but also, many suspected, to prepare another Presidential bid in 1988.

Alas, another White House scandal upset those plans. In 1987 Reagan summoned him back to the White House as Chief of Staff to steady an administration reeling from the Iran-Contra affair, in which arms were sold to Iran and the proceeds used to support the Nicaraguan Contra rebels, in defiance of every Congressional rule. Having assured himself that this time the president truly did know nothing of what his underlings were up to, Baker accepted the offer. And the ship was quickly steadied. Reagan's approval ratings climbed steadily through his remaining two years in the White House, and Baker's reputation as the supreme Washington operator was sealed.

In July 1988, the job done, Baker left the White House. But his public service was not over. In 2001 he accepted President George W Bush's offer to become the US Ambassador to Japan, where he served for four years. In 2007 he and three other former Senate leaders founded the Bipartisan Policy Center, promoting cross-party solutions to the country's problems. But the venture sank virtually without trace. American politics had changed utterly since Baker's heyday – and indubitably for the worse.

Howard Henry Baker, politician: born Huntsville, Tennessee 15 November 1925; US Senator, Tennessee 1967-1985; Republican majority leader 1981-1985, White House chief of staff 1987-1988, US Ambassador to Japan 2001-2005, married firstly Joy Dirksen (deceased; two children), secondly Nancy Kassebaum; died Huntsville 26 June 2014.