The Labour candidate selected to contest the crunch Stoke Central by-election battle claimed Jeremy Corbyn was an "IRA supporting friend of Hamas".
Cllr Gareth Snell made the comments on Twitter as the party's leadership contest between Mr Corbyn and Owen Smith was in full swing in July last year.
Mr Snell said: "If we have to prefix (Owen) Smith with 'Pfizer lobbyist', can we also say 'IRA supporting friend of Hamas career politician' Jeremy Corbyn?"
A spokeswoman for Mr Corbyn on Thursday refused to say whether he had been aware of the tweets.
"We won't comment on this," the spokeswoman said.
Mr Snell will attempt to hold the Stoke-on-Trent Central seat for Labour next month.
He is taking on Ukip leader Paul Nuttall, who will hope to capitalise on a strong Brexit vote in last year's EU referendum and secure himself a place in the Commons.
Borough councillor Mr Snell - standing for the seat in a city where overall almost 70% voted for Leave - also once tweeted that Brexit was a "massive pile of s**t".
The by-election was triggered after Tristram Hunt announced he would be stepping down to become director of the Victoria and Albert Museum.
In a series of tweets on July 19, Mr Snell said: "We doing hyberbole? If so, it's a choice between IRA supporting friend of Hamas career politician Corbyn vs Smith."
He also said: "Smith learnt politics in the heat of the fight in South Wales. Corbyn sipped tea in trendy north London with career politician and Viscount."
Later, Mr Snell added: "For clarity, my previous comments were to demonstrate the absurdity of the hyperbole I've read in the last two hours."
Mr Corbyn won the leadership contest on September 24 last year.
In a separate tweet, on September 29 2016, Mr Snell said: "Soft Brexit, Hard Brexit, massive pile of s**t. Sloppy Brexit, messy Brexit, quit, quit, quit."
Meanwhile, the Conservatives selected project manager Trudy Harrison to take on Labour in the Copeland by-election.
The two contests on February 23 will present a headache for Jeremy Corbyn, with strong challenges to Labour in both seats.
In the 2015 general election, Mr Hunt had a 5,179 majority over Ukip - with the Tories third.
After announcing his candidacy - but before the Twitter story broke, Mr Snell said: "I'm delighted to have been chosen by local Labour members here in Stoke-on-Trent Central to be the voice of local people.
"I'm proud to come from the Potteries and even prouder to be standing to represent our home in Parliament.
"Labour has always stood up for local people here in Stoke-on-Trent, whether it's building our first new hospital in 100 years or defending our vital children's centres.
"The Tories haven't done anything for our city and now they're trying to sell off £1.2 billion of cancer and end-of-life care in North Staffordshire.
"And Ukip's plans for our NHS are clear - sell it all off to the highest bidder.
"Only Labour will do what is needed to get a fair deal for the people of Stoke-on-Trent Central."
The Tories have selected 25-year-old councillor Jack Brereton to stand in Stoke Central .
He said: "This by-election has come at a crucial time for our country. It is an opportunity for the people of Stoke to send a message that the referendum result must be respected, and to back the Prime Minister's clear plan to make a success of Brexit."
In Copeland, mother-of-four Ms Harrison will attempt to snatch the seat from Labour after Jamie Reed quit to take up a position at the Sellafield nuclear power plant.
Labour's majority was cut to 2,564 at the last general election, and the Tories hope to take the seat by highlighting their commitment to nuclear power and appealing to the Brexit-supporting local electorate.
Ms Harrison, 40, said: "Copeland has had Labour MPs and Labour councils for years. They've ignored us and failed to deliver the jobs, infrastructure and services we need, and now they want to ignore how we voted in the referendum.
"I look forward to meeting as many local residents as possible in the coming weeks and setting out how voting Conservative this time will support our local nuclear industry, and deliver the investment Copeland deserves."