With Labour leader, Jeremy Corbyn, entering the fray at the 11th hour to take part in what turned out to be a seven-strong debate on the BBC on Wednesday (31 May), May's absence was a recurring motif for the other politicians and for Twitter users.
Mrs May, who backed Remain in the referendum, will say she believes Brexit provides an opportunity to make the UK more global and outward-looking - a Britain alive with possibilities, more confident in itself, more united and more secure, a country our children and grandchildren are proud to call home.
"Where do you think Theresa May is tonight?"
YouGov said May was still the most favoured choice for prime minister, though her 43 percent rating is the lowest it has ever been.
Ms Rudd dismissed her rivals' claims on public spending as "fanciful" and warned their plans would require a "magic money tree" as she came under attack over the squeeze on living standards and cuts to welfare.
"I've been taking Jeremy Corbyn directly week in and week out at Prime Minister's Questions", she said. She can't be bothered. But Liberal Democrat leader Tim Farron, whose party has struggled for attention during the campaign, was the first to draw attention to the prime minister's absence.
Speaking on security, UKIP leader Paul Nuttall said while British foreign policy in Iraq and Libya were "fundamentally wrong", they were not to blame for the attack in Manchester that killed 22 people.
Mr Letts summed it up as a "truly terrible debate", adding: 'Had I not been watching it for your delectation, I would have turned off half way through'.
His spokesman said May's "refusal to take part showed her weakness" as she "won't even debate her opponents here in the United Kingdom, in an election she called".More news: Congress Subpoenas Trump And Obama Officials In Russia Inquiry
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On Brexit, Mr Corbyn said Labour would negotiate tariff-free access to the single market to protect manufacturers.
"Not the iron lady, more the u-turn queen", he said.
The Prime Minister, who was ridiculed by opponents after refusing to take part in a major TV debate on Tuesday night, will suggest there could be a "brighter future" for the United Kingdom if there is a good deal with the European Union.
Ms Rudd was laughed at by the audience as she called for people to "judge us on our record" on the public finances.
Mrs May was heavily criticised for her decision to miss the debate, but Boris Johnson defended her. I believe that with determination, ingenuity and common sense, we can use this moment of great national change to shape a better future for Britain. "Don't give up on me yet, Angus", she said. She added: "Why is she such a coward?"
Mr Corbyn defended his speech last week linking the Manchester bombing to Britain's military interventions overseas, winning support from Mr Robertson and applause from the audience.
Ms Rudd said the central question in the election was who should negotiate with the European Union on behalf of Britain, Theresa May or Jeremy Corbyn.
Mr Corbyn responded that Mrs May and other senior Conservatives had voted against anti-terror laws in 2005. You are not worth Theresa May's time. "Don't give her yours".