Reform UK leader Richard Tice has warned the country faces "Starmergeddon" if it elects Labour, as he called for a general election.
The rebranded Brexit Party, which is on about 9% in the polls, has previously focused its attacks on the Tories.
But Mr Tice claimed a government led by Sir Keir Starmer would be a "disaster".
He ruled out an electoral pact with the Tories - and said he was "confident" Nigel Farage would play a role in Reform's campaign but did not say how.
Sir Keir is due to make his own new year speech on Thursday, as Labour steps up campaigning ahead of a general election, expected as soon as the spring.
There has been speculation Mr Farage, who is Reform UK's honorary president and helped found the party in 2018, could make a return to front-line politics after his stint in the I'm A Celebrity jungle.
However, he was not among the "special guests" promised at Reform's new year press conference and Mr Tice said the former UKIP leader said was "still assessing" the extent of the role he would play in helping the party.
Mr Farage is not expected to make another bid to become an MP after failing to get elected to the UK Parliament seven times, but could be given a campaigning role.
Mr Tice used his speech to criticise both Labour and the Tories, accusing them of betraying working class voters on immigration.
In an attempt to tap into a sense of disillusionment among some Tory voters, Mr Tice claimed the government had "opened the borders to mass uncontrolled immigration of a scale that this country has never seen before".
Meanwhile, he said the country faced "a catastrophic cocktail of economic incompetence" if Labour won power.
Mr Tice added: "Only Reform UK is now the party of the working class, who will stop mass immigration, who will scrap net zero, who will help solve the cost of living crisis."
Setting out a number of the party's key policies, he said Reform supported increasing the threshold when people start paying income tax to £20,000, as well as cutting fuel duty and taxes for businesses.
He called for a freeze on non-essential immigration and a "one-in, one-out" policy.
At the press conference, Mr Tice also revealed former MEP Ben Habib as Reform's candidate for the Wellingborough by-election, which was triggered by the recall of former Tory MP Peter Bone.
Mr Tice said the public wanted an election "sooner rather than later" as voters wanted to "punish the Tories for breaking Britain".
Earlier, the Liberal Democrats called for an election to be held in May, accusing Prime Minister Rishi Sunak of "clinging on to power".
A general election must take place by the end of January 2025 but is expected this year.
Mr Tice repeated his insistence that Reform would stand candidates in every single seat across England, Scotland and Wales, saying he was "absolutely categoric" there would be no deals with the Tories.
In 2019 Reform UK's previous incarnation, the Brexit Party, stood aside in more than 300 seats previously won by the Tories, amid concerns it could split the pro-Brexit vote.
Mr Tice denied that by standing in every seat Reform risked enabling a Labour government, adding: "The best way to avoid Starmergeddon is for everybody to vote for Reform UK because only our policies can save Britain."
A recruitment drive has been underway to find candidates, with Mr Tice claiming nearly 500 had been approved.
However, running such a widespread campaign takes resources and could prove a challenge without the banner of Brexit flying as high as it did in 2019.
Reform is currently just behind the Lib Dems in opinion polls and some Conservatives are worried the party could capitalise on concern over legal and illegal migration to pick up votes from disaffected Tory supporters.
Tory deputy chairman Lee Anderson has described Reform as "a bigger threat to the country" than the Labour Party.
"If Reform do pick off a lot of us Conservative MPs at the next election, then what's going to happen is we're going to end up with a Labour government," the MP for Ashfield, in Nottinghamshire, told GB News on Tuesday.
However, he said if the party wanted to make any inroads at the next election, Mr Farage needed to become leader and stand as a candidate.
Mr Tice brushed off the comments, saying Mr Anderson was "terrified that we're going to basically put him out of the job".
