Hegseth has been a polarising figure in American politics and has made enemies, even within his own Republican Party. One Republican lawmaker, US Representative Thomas Massie, even filed articles of impeachment against him earlier this year, accusing him of abusing his office by launching hostilities against Iran without approval by Congress.Republican US Senator Thom Tillis told CBS on Tuesday night that Hegseth has "a reckoning" coming and his strained relationship with lawmakers was undermining the Trump administration's push for a US$1.5 trillion defence spending package in Congress."I think after the election, we need to determine whether or not he's the best person to bring those needs to Congress if he wants something done before the end of this Congress, which is only 60 days away," Tillis said as the midterm results came in.Democrats have promised to investigate Hegseth and his handling of the Pentagon, and aides say that could include everything from his handling of the Iran war to his purges of top military officers, including the chairman of the Joint Chiefs of Staff.He could also face scrutiny over his Ukraine policy and US military strikes on suspected drug trafficking boats in Latin American waters, which some Democrats say could amount to war crimes.At times, the conflict between Hegseth and members of Congress has become personal. Last year, Hegseth sought to prosecute Democratic US Senator Mark Kelly after he and five other Democrats in Congress with backgrounds in the military and intelligence urged US troops to refuse illegal orders.Hegseth accused them of sedition.Representative Adam Smith, the Democrat who is expected to lead the House Armed Services Committee next year, promised vigorous oversight on everything from the Iran war to what he called the erosion of the "apolitical foundation of the armed forces".A source familiar with the matter said rancour on Capitol Hill over Hegseth has featured in White House discussions about potential replacements for him, saying there would be a need to have someone in the job who could distance themselves from events under Democratic scrutiny.Chief Pentagon spokesperson Sean Parnell said Hegseth would continue to drive Trump's agenda and that the Pentagon "has followed the law"."Secretary Hegseth will not back down or be deterred by politicians seeking to score partisan points or launch phoney investigations where no wrongdoing exists," Parnell said.