President Donald Trump vowed a political comeback on Sunday after his Republican Party lost control of Congress in last Tuesday's midterm elections, as incoming Democratic leaders promised to hold his administration to account and the nation's top law enforcement official pledged to defend officials against threatened prosecutions.

The heavy defeat leaves Trump politically hamstrung as he enters the remaining two years of his White House tenure, a stretch known as the lame duck period. It also opens the door to multiple investigations by the Democrat-controlled Congress into the president, his family and senior members of his administration.

Writing on Truth Social, Trump said, "America took a giant step back in History with the results of the last Election," noting that the stock market, which had grown during his tenure, took a loss after the vote. He added: "But have no fear, we will be back, BIGGER AND STRONGER THAN EVER BEFORE!"

On Friday, a defiant Trump appointed hardline advisor Stephen Miller as his new chief of staff, signaling a combative stance even though many Republicans, including Vice President JD Vance, have reflected on why the party lost so badly at the ballot box. Miller was the architect of Trump's tough immigration policy, which analysts have identified as a contributing factor to the unpopularity of Republicans and their election debacle.

Trump also said on Saturday that he was ordering the Justice Department to examine Democratic threats to impeach members of his administration, likening the threats to extortion.

With control of committees in both chambers of Congress, Democrats can summon officials, demand documents and hold public hearings, placing Trump and his senior aides under intense scrutiny. Likely subjects include the Trump family's business and cryptocurrency dealings, government contracts involving billionaire Elon Musk, cuts to federal agencies, immigration enforcement and the conduct of the war in Iran.

Hakeem Jeffries, who is set to become speaker of the Democrat-controlled House of Representatives, told ABC he would support an effort to oust Defense Secretary Pete Hegseth, a key architect of the war on Iran that began in February. "I'm a hard yes on the Hegseth resolution," Jeffries said when asked about a Republican lawmaker's reported plan to introduce an impeachment motion as early as Monday, adding that Hegseth had abused his power, undermined national security and engineered a reckless and costly war of choice in Iran. "Pete Hegseth has got to go."

Speaking with reporters on Sunday, Trump defended Hegseth, saying: "I mean he's guilty of two things, basically winning two wars," a reference to the January US raid on Venezuela that removed president Nicholas Maduro and the ongoing war with Iran. "We won in Venezuela. We've taken out hundreds of billions of dollars. We won in 48 minutes. And we won in Iran," Trump said. "It's just a question of closing it out now or later."

Attorney General Todd Blanche, a former personal lawyer for Trump who now heads the Justice Department, vowed to actively defend the president and senior members of his administration. "If they try to come after Stephen Miller, instead of actually just conducting oversight and leading, of course the Department of Justice will be there to defend him," Blanche told CNN.

Robert Garcia, the ranking Democrat on the House Oversight Committee, took aim at Blanche, saying one role of Congress under the US constitution is to hold the executive branch to account. "All he wants to do is protect Donald Trump, is protect Donald Trump's cabinet and administration officials. That is not his job," Garcia told CNN, vowing to stop corruption and investigate "those that are committing crimes against the American public." He added: "They can come after us however they want. We're not going to be intimidated."