President Donald Trump ordered the US Justice Department on Saturday to look into Democrats for “threatening a lot of people” with impeachment and other ​forms of punishment, saying in a social media post that their actions amount ‌to a “criminal act”, without providing evidence.

Democratic candidates routed Trump’s Republicans in midterm elections on Tuesday, giving them control of Congress starting in January. Party leaders have promised to reassert Congress’ oversight of the executive branch once ​they are in power by holding hearings and conducting investigations into Trump administration actions.

In ​his post, Trump accused Democrats of sending “threatening letters to many people, all innocent”.

Members ⁠of Congress have released copies of letters seeking documents on matters such as tech company donations ​for the large ballroom Trump is building at the White House.

Trump’s post also referred to Democratic ​Senator Elissa Slotkin of Michigan, who suggested in remarks last month at the University of Chicago that Congress should investigate people who have enabled Trump’s policies.

“I want young people coming out of school to say, ‘Should I ​go work for the Trump administration?’… I want them to be like, ‘Lord, that might be ​buying myself a ton of problems that I don’t want. And so I might avoid participating in that,’” Slotkin said.

“I ⁠want that deterrence message sent.”

Her comments, within weeks of the midterm elections, were criticised by some Republicans. Republican Senator Mike Lee of Utah wrote on social media that Slotkin was threatening young people who wanted to go into politics. Slotkin responded that Lee had lied about her comments ​and wrote, “Spare us the ​manufactured outrage”.

Trump’s Justice Department ⁠has investigated Slotkin before. A former CIA analyst, she was among a group of Democratic lawmakers with national security backgrounds whom the department investigated ​after they posted a video urging service members and intelligence officials to ​disobey illegal ⁠orders from the administration.

A grand jury rejected the attempt to indict them.

Slotkin said in a social media post on Saturday that the issue should not be controversial. “If you break the law or use ⁠taxpayer dollars ​to enrich yourself, you should be held accountable. If ​you didn’t break the law, then it’s all good,” she said.

The Justice Department did not immediately respond to requests for comment.