Former candidate to be Democratic nominee for US president records video backing Green party’s Larry Sanders in Witney byelection

US presidential hopefuls do not often intervene in British parliamentary byelections. But then, not many presidential candidates are Bernie Sanders. And more than that, he is intervening on behalf of his older brother.

Sanders, who missed out on taking the Democratic nomination from Hillary Clinton, has recorded a brief but heartfelt campaign video aimed at voters in Witney, Oxfordshire, that talks up the attributes of his brother, Larry.

Larry Sanders, who has lived in the UK since 1969, is standing for the Green party in the constituency, which will elect a new MP on 20 October to replace David Cameron, who quit the Commons last month.

Bernie Sanders tells viewers in the video “that I do not know a heck of a lot about British politics”, adding: “But I do know a lot about my brother, Larry Sanders.”

Describing his brother’s “very strong influence over my life”, Sanders said: “From when I was a very little kid, Larry and I discussed issues of social justice, economic justice, and racial justice. And what I can tell you is that my brother is a very, very caring human being, who wants to see government represent all of the people, not just the people on top.”

He added: “He wants to see a society in which we do not have massive levels of income and wealth inequality, wants to see a society where healthcare is a right of people and that means quality healthcare under the national health system.

“For decades now, and I hear this from him almost every week, he has been working to strengthen the NHS to ensure that it is the quality system that everyone in the UK is entitled to.”

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Larry, the Oxford-based Greens’ spokesman on health matters, has been a local councillor and spent eight years as leader of the county council’s Green group.

The Green party and the Liberal Democrats are making considerable efforts to produce some sort of upset in Witney, where the Conservative candidate, councillor Robert Courts, remains the strong favourite. Cameron won just over 60% of the vote in the 2015 election.

Jonathan Bartley, the Greens’ co-leader, welcomed Bernie Sanders’s endorsement. He said: “With Larry as a candidate the people of Witney have a chance to vote for someone putting forward the bold policies we so desperately need to build a better Britain.”

Also on Tuesday, Nick Clegg campaigned in the constituency with the Lib Dem candidate, Liz Leffman, another councillor. The party is hopeful of improving on its distant fourth place in Witney in 2015, with the party’s leader, Tim Farron, having visited three times during the campaign.

• This article was amended on 14 October 2016. An earlier version referred to the Lib Dems’ “distant third place in Witney in 2015”; they came fourth in that election, behind the Conservative, Labour and Ukip candidates.