Mike Gravel, the iconoclastic two-time Democratic US senator from Alaska, who has died aged 91, was best known for the day in 1971 when, in a meeting of the Senate subcommittee on building and grounds, he read for three hours from the Pentagon Papers, and put the entire document into the congressional record.

The papers, a 7,000-page official study of the Vietnam war, which contradicted virtually everything the public had been told by successive governments, had been leaked to newspapers by one of its authors, Daniel Ellsberg, but the Nixon administration had won an injunction against their publication.

The day after Gravel’s reading, the US supreme court, in New York Times Co v United States, quashed that prior restraint, and the papers were published, including Gravel’s own copy, by Beacon Press.

Although he opposed much of US policy abroad, Gravel was also a business-oriented politician, whose major legislative accomplishment in the Senate may have been his exempting the trans-Alaska oil pipeline from the requirements of the National Environmental Policy Act of 1969 introduced by the powerful Democratic senator Henry Jackson.

Gravel’s exemption of 1973 needed a casting vote by the Republican vice-president Spiro Agnew to pass. Gravel could be a divisive force in his own party, and after his Senate career ended was often dismissed in Washington as a gadfly, but his shifting positions on the left-right spectrum were not unusual in Alaskan politics, where he also needed to overcome the idea that he remained an outsider.

Born in Springfield, Massachusetts, he was the son of French-Canadian immigrants, Alphonse Gravel, a builder, and Marie (nee Bourassa), and spoke French at home in his early years. He struggled at school – Assumption prep, in Worcester – and at 18 he decided to join the Israeli army fighting in Palestine.

In New York, seeking advice on getting to Israel, he met Alexandra Tolstoy, daughter of the novelist, who was involved in helping Russian immigrants. She told him to finish school. He returned to Assumption, where an English teacher helped him cope with dyslexia and coached him to graduation.

After a year at Assumption college, and two at American International college back in Springfield, he faced the Korean war draft, and enlisted in the army. He served in Germany and in France, where his knowledge of French saw him assigned to spy on the French Communist party.

After his discharge, he gained an economics degree (1956) from the school of general studies at Columbia University, New York. Moving to Alaska, not yet a state, he worked on the railways, sold real estate and became active in the Democratic party. In 1958 he lost his first election campaign, for the territory’s house of representatives. The following year he married Rita Martin, and went into property development. That year, too, Alaska joined the union.

In 1962, his firm went bust, but he was elected to the state house, serving as speaker in his second term. In 1968 he entered the US Senate primary against Ernest Gruening, one of only two senators (along with Oregon’s Wayne Morse) to have voted against the Tonkin Gulf Resolution that authorised President Lyndon Johnson to fully involve US forces in Vietnam. Gravel positioned himself as a supporter of the war effort. He won the primary, and despite Gruening running as an independent, then won a three-way race for the Senate.

In Washington, Gravel established himself as a critic of the war, twice fighting extensions of the military draft, including once by filibuster. He worked against allowing nuclear testing in Alaska, but also opposed legislation to designate massive amounts of Alaskan land as national parks protected from development. As well as joining Republicans to pass the pipeline, he aligned with conservative southern Democrats to preserve the filibuster they cherished to protect “states’ rights”.

In 1972, Gravel published Citizen Power: A People’s Platform, detailing his positions on all major issues. When the presidential candidate George McGovern wanted to have the Democratic convention select his vice-president by a vote, Gravel added to the chaos by nominating himself. McGovern eventually selected Senator Thomas Eagleton as his running-mate (although after revelations he had been treated with electric shock therapy for depression, Eagleton was forced to withdraw).

After winning a second term in the Senate in 1974, Gravel faced scandals when a staff memo detailing plans to raise funds from oil companies was leaked, and when he was accused of having been set up in a “sex for votes” scandal (he admitted having the sex, but denied changing a vote), which also cost him his marriage. He was defeated in the 1980 Senate primary by Clark Gruening, Ernest’s grandson, with the help of Republican votes under Alaska’s open primary system.

After the Senate, Gravel’s career as a property developer did not flourish; he lost his Senate pension in his 1981 divorce. In 1984 he married Whitney Stewart, an aide to the New York senator Jacob Javits, and her money helped support the couple. Gravel began a foundation to support direct democracy, through referendums, then became chair of the Alexis de Tocqueville Foundation, with similar aims.

In 2006 Gravel announced his candidacy for the 2008 presidency, and in the early democratic primary debates stole the show, arguing that US foreign policy was neither altruistic nor defensive in nature. The attention did not translate into funding or votes. He switched to the Libertarian party, to which by now he seemed more naturally attuned, with what was becoming his increasingly populist position, but failed to win their nomination.

Although he made gestures toward the 2016 and 2020 Democratic presidential races, his efforts were hamstrung by his propensity to take the positions, on everything from relations with Iran to UFOs and 9/11 conspiracies, that pushed him into gadfly territory.

He became chief executive of a company producing medical marijuana, and in 2018 published an updated edition of People’s Power. In 2020 he used his remaining campaign funds to found the Gravel Institute to promote progressive politics.

He is survived by his wife and a son, Martin, and daughter, Lynne, from his first marriage.