British Prime Minister Boris Johnson and opposition Labour Party leader Jeremy Corbyn traded blows Tuesday over Brexit and Donald Trump at the onset of a bruising pre-Christmas election campaign.Johnson's main rival in the snap December 12 poll has struggled to explain his position on Brexit ever since Britons narrowly triggered the divorce in a 2016 referendum.Labour's new official stance is to negotiate a more EU-friendly withdrawal agreement with Brussels and then let voters decide whether to back it or simply stay in the EU.But Corbyn refuses to say whether he would then campaign for his own deal. Most top members in the party oppose Brexit and have said they would campaign to remain in the bloc.Johnson's Conservatives are trying to emphasise Labour's divisions on the defining issue in contemporary British politics."Now the time has come for you to come clean," Johnson told Corbyn in a letter released by his office."Do you believe the results of the 2016 referendum should be respected and the UK should leave the EU?" Johnson asked."Would you commit to campaign for your 'deal' in a second referendum?"'Clear and simple' Corbyn shot back that Labour was putting back to the people an issue that politicians have been unable to resolve for more than three years."It's really not very complicated," Corbyn told his supporters."The Tories have failed on Brexit for three years. A Labour government will get Brexit sorted within six months by giving you, the British people, the final say," the veteran socialist said."And despite what some commentators want you to believe, Labour's plan for Brexit is clear and simple."He accused Johnson of selling out British interests to US President Donald Trump in an effort to strike a lucrative post-Brexit trade agreement."They demand alignment," Corbyn said in reference to Washington demands for London to lower its food safety standards to allow US agricultural firms to compete on the UK market."And you will see the price of it... on the pressures of our economy and the rights of people who work for those companies."Corbyn has been a trenchant critic of Trump and he turned down an invitation to attend a gala dinner thrown by Queen Elizabeth II during the White House chief's visit to London in June.Labour is trailing the Conservatives by 11 percentage points in a poll of polls compiled by Britain Elects.But the field also includes smaller pro-EU opposition parties that could potentially form a post-election coalition with Labour -- if they ever agree on who should lead the government.The anti-Brexit Liberal Democrats came in second in European Parliament polls in May that Britain was forced to take part in because of Brexit delays.Lib Dem leader Jo Swinson formally launched her party's campaign on Tuesday in London, with it currently running in third place with around 18 percent of the vote.But she firmly refuses to back Corbyn - the official leader of the opposition - and wants to become prime minister herself."I am absolutely, categorically ruling out Liberal Democratic votes putting Jeremy Corbyn into Number 10," Swinson said."On the biggest issue of the day, he has prevaricated and will not give a straight answer - even now, when you will ask him if he is Remain or Leave -- he will not tell you how he will vote," she told reporters.The Brexit Party of populist Nigel Farage is currently polling 11 percent and will potentially pile pressure on Johnson during the campaign in the hope of luring votes from the anti-European right.