US senators clashed over the need for key witnesses to testify at Donald Trump’s impeachment trial, ahead of Tuesday’s start to the main proceedings to decide whether the president should be removed from office. 

The stand-off over public testimony by officials with knowledge of Mr Trump’s actions related to the Ukraine scandal — which is the subject of the articles of impeachment against Mr Trump — has become the dominant political flashpoint between lawmakers in recent weeks.

Democrats have been demanding that the Republican majority allow key officials, such as John Bolton, the former national security adviser, and Mick Mulvaney, the White House chief of staff, to appear at the trial. Mitch McConnell, the Senate leader, and the White House, are opposed, but some centrist Republicans are sympathetic to the idea, putting pressure on Mr Trump’s party to give in.