Nick Clegg has warned David Cameron not to use Ukip as an "excuse" to duck 2010-style TV election debates.
The Lib Dem leader issued the challenge as he announced he was ready to sign up to another round of televised argument.
The Prime Minister is now the only one of the three main party leaders at Westminster who has not committed to taking part. Mr Cameron has suggested he could be persuaded if the televised debates were organised in a different format to four years ago.
Specifically, he wants them spread out over a longer period of time and not held in the final few weeks of the campaign.
Separately, there have also been calls for Nigel Farage, whose Ukip party is currently doing well in UK-wide polls, to take part.
But others have argued that smaller parties who already have sitting MPs, such as the SNP and the Greens, should take precedence over Ukip, which currently has no politicians at Westminster.
Held for the first time in 2010, the debates were a phenomenon, attracting high viewing figures and sparking "Cleggmania" - a brief surge in Lib Dem support.
Labour leader Ed Miliband has already said that he is prepared to take part in 2010-style TV debates.
Yesterday Mr Clegg echoed that view, saying: "I'm ready to sign up now - as leader of the Liberal Democrats - to the same format as happened last time."
He added that he hoped Mr Cameron would also agree to take part and indicated he was relaxed about any potential inclusion of Mr Farage.
Mr Clegg said: "I hope (the Conservatives) won't use their anxieties about Ukip and Nigel Farage as an excuse not to give the British people the right to see those leaders' debates again next time.
"I think it would be a real step backwards if any party leader were to use an excuse not to do these leaders debates again."
Meanwhile, Nick Clegg has challenged the Conservatives to give 20 million workers £140 by scrapping plans for tax breaks for married couples.
He called on the measure to be included in the next Budget, on March 19. Under his proposals, ministers would increase the personal income tax allowance to £10,500, giving what he described as a "workers' bonus".
But he said that threshold could be raised to £10,700 - if the Tories ditched "pet tax projects" such as the married couples' tax allowance. Speaking at a press conference in Westminster, Mr Clegg said: "My priority in the Budget, in a few weeks' time, will be to deliver a 'workers' bonus'."
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