US President Donald Trump has that Democrats are planning to meddle in the 2018 midterm elections, and called on Republicans to “stay tough” in the coming months.

“The 13 Angry Democrats (plus people who worked 8 years for Obama) working on the rigged Russia Witch Hunt, will be MEDDLING with the mid-term elections, especially now that Republicans (stay tough!) are taking the lead in Polls,” Trump tweeted on Tuesday.

“There was no Collusion, except by the Democrats!” he added.

Trump's tweet is the latest in a series of posts lashing out at special counsel Robert Mueller's team of investigators – whom he has dubbed the “13 angry Democrats” – investigating possible collusion between his presidential campaign and Russia in the 2016 election.

Trump has said that his election campaign’s collusion with the Russian government during the 2016 election was a “hoax”.

He has repeatedly denied allegations that his campaign colluded with Russians and has condemned the investigations. Russia has also denied the allegations. Trump has also labeled Mueller’s investigation as a “witch hunt,” and has suggested that Russians might have meddled in favor of Clinton.

Trump has also said that those working on Mueller's investigation are biased against him, and that the probe is a Democratic effort to undermine his presidency.

“Why aren’t the 13 Angry and heavily conflicted Democrats investigating the totally Crooked Campaign of totally Crooked Hillary Clinton,” Trump tweeted. “It’s a Rigged Witch Hunt, that’s why! Ask them if they enjoyed her after election celebration!”

In recent weeks, Trump has also said that Democrats planted a mole in his presidential campaign in 2016.

Trump said in a tweet on Friday that the FBI had used a top-secret informant to spy on his team long before the bureau started investigating the alleged Russian meddling in the 2016 election.