Friday's televised showdown comes amid an ongoing controversy over Johnson's decision to avoid an in-depth interview with Andrew Neil, a BBC journalist known for his forensic questioning. Four other party leaders, including Corbyn, endured such a grilling, and Neil has accused Johnson of "running scared.''
Neil issued a challenge to Johnson on national television Thursday, saying political leaders in the last two U.K. elections had agreed to be interviewed by him: "All of them. Until this one."
He said Johnson needed to answer questions about trust, "and why at so many times in his career, in politics and journalism, critics and sometimes even those close to him have deemed him to be untrustworthy."
Cabinet minister Michael Gove, a Johnson ally, urged voters to call 10 Downing St. and ask whether Johnson would agree to the interview.
He recited the number on LBC radio and said "if you ring the prime minister's diary secretary, he will know, or she will know, what the prime minister is going to do."
Johnson shrugged off the pressure, insisting he had done plenty of interviews during the campaign, and appeared to lump Neil in with a joke candidate in the election.
"We cannot accommodate everybody," he said. "There's guy called Lord Buckethead who wants to have a head-to-head debate with me. Unfortunately, I'm not able to fit him in."
Johnson, a former mayor of London who helped lead the campaign to take Britain out of the European Union, has long faced questions about his character. As a journalist, he was once fired for fabricating a quote. In politics, he was sacked as party vice chairman for lying about an extramarital affair. In a magazine article last year he called Muslim women who wear face-covering veils "letter boxes." Authorities are investigating his relationship with American tech entrepreneur Jennifer Arcuri, who allegedly received favors and public funds while Johnson was the mayor of London. But Johnson has insisted that "everything was done with full propriety."
And yet opinion polls put Johnson's Conservatives ahead of the Labour opposition ahead of the election next Thursday, in which all 650 House of Commons seats are up for grabs. The Tories are keen to avoid any slip-ups that could endanger that lead.
The Conservatives had a minority government before the election, and Johnson pushed for the December vote, which is taking place more than two years early, in hopes of winning a majority and breaking Britain's political impasse over Brexit. He says that if the Conservatives win a majority, he will get Parliament to ratify his Brexit divorce deal and take the U.K. out of the EU by the current Jan. 31 deadline.
Labour has promised to negotiate a new Brexit deal, then give voters a choice between leaving on those terms and remaining in the bloc. It also has a radical domestic agenda, promising to nationalize key industries and utilities, hike the minimum wage and give free internet access to all.
The party has struggled to persuade voters that its lavish spending promises are deliverable without big tax hikes. Labour's campaign also has been dogged by allegations that Corbyn — a long-time champion of the Palestinians — has allowed anti-Jewish prejudice to fester in the left-of-center party.
Corbyn has called anti-Semitism "a poison and an evil in our society" and says he is working to root it out of the party.
This election is especially unpredictable because the question of Brexit cuts across traditional party loyalties. For many voters, their identities as "leavers" or "remainers" are more important than party affiliations.
The Conservative lead suggests the party has managed to win over many Brexit-backing voters, while Labour faces competition for pro-EU electors from the centrist Liberal Democrats and several smaller parties.
But the Conservatives have also lost support from some pro-EU voters by taking a strongly pro-Brexit stance. Several ex-Conservative lawmakers who were expelled for rebelling over Brexit are running against their old party as independents.
The independent former Tories were endorsed Friday by former Conservative Prime Minister John Major, who called Brexit the "worst foreign policy decision in my lifetime."
"It will make our country poorer and weaker," he said. "It will hurt most those who have least."