Kamala Harris chooses Minnesota governor Tim Walz as her running mate against Trump and Vance in US elections

Since one job a vice-presidential candidate has traditionally held in US politics is to be a sharp critic of the opposing ticket, his effectiveness during interviews on cable news shows helped bring him to the Harris campaign’s attention.

Harris, who last week secured enough votes from Democratic delegates to become the party’s presidential nominee, will face Republican nominee Donald Trump and his running mate, Senator J.D. Vance of Ohio, who have struggled to respond to strong support for Harris since President Joe Biden announced last month that he was withdrawing from the campaign and threw his support behind his vice-president.

Harris is the first woman of colour at the top of a major party’s ticket, a possible liability in the Democratic Party’s efforts to win over conservative voters, and a key reason that she had been expected to choose a white man as her running mate.

Republicans have also labelled Harris a “California liberal”, giving her more reason to choose someone from a Republican-leaning or swing state to join her.

It remains to be seen whether Walz gives Harris more momentum in polls, which have shown her roughly even with Trump, and in some cases slightly ahead. Trump’s pick of Vance has been widely seen as dragging the former president’s standing in the polls, giving rise to talk that he may switch to another running mate.