The Liberal Democrats have ruled out propping up a Labour government if the General Election delivers another hung Parliament.
Party leader Jo Swinson said Jeremy Corbyn was "not fit for the job of prime minister", and warned he would be a "threat to our national security".
Setting out her stall to lead the country, she said the December poll could be a moment of "seismic change" where a "new and different" politics could emerge.
It came as Mr Corbyn said he wanted to get Brexit "sorted" within six months if his party came to power.
He accused Boris Johnson of trying to "hijack" the UK's withdrawal to "sell out" the NHS and working people, and said the Conservatives want a "race to the bottom in standards and protections".
Launching her party's campaign in Westminster, Ms Swinson said she is "absolutely categorically ruling out" Liberal Democrat votes putting Mr Corbyn into Number 10.
"On so many grounds, Jeremy Corbyn is not fit for the job of prime minister.
"On the biggest issue of the day, he has prevaricated and will not give a straight answer.
"Even now if you ask him whether he is Remain or Leave he will not tell you how he would vote.
"His plans for the economy would take us back to the 1970s. I believe he would be a threat to our national security."
Mr Corbyn set out his Brexit plan during a speech in Harlow after the Prime Minister said voters deserved to have a "clear picture" of what potential leaders will do about it.
The Labour leader told supporters his party's plan was "clear and simple", adding: "It's time to take the decision out of the hands of politicians and trust the people to decide.
"It won't be a rerun of 2016. This time the choice will be between leaving with a sensible deal or remaining in the European Union. That is the policy. It really isn't very complicated."
He said the June deadline for getting a Brexit deal was a "realistic" target, and that the party would work in "parallel" with Parliament to legislate for a referendum on the Labour deal within six months.
"The deadline we've set is a realistic one. We wouldn't be saying it if we didn't think it was do-able and possible."
Shadow Brexit secretary Sir Keir Starmer, speaking before Mr Corbyn, said the Government's Brexit deal would leave the UK "£70 billion worse off".
"If Boris Johnson wins this election our country will take a decisive lurch to the right. His Brexit deal is a hard right deal," he said.
Mr Johnson wrote to Mr Corbyn on Monday night saying his rival had "sought to avoid explaining" what his Brexit plan is, and seems to want to "go back to square one".
He wrote: "When they choose the next Prime Minister, the voters deserve to have a clear picture of what each potential leader will do when it comes to Brexit."
The PM told his Cabinet on Tuesday morning that ministers should be "very proud" of their achievements ahead of the General Election which he said "none of us particularly wanted to hold".
"I think we wanted to get on with delivering for our country and getting on with delivering the fantastic things that we've been doing over the past 100 days," he said.
"I think this Government, in its short period in office, can be very proud of what it's done. We are levelling up across the country with infrastructure, education and new technology."
Elsewhere, Nigel Farage kicked off his Brexit Party's nationwide tour as part of its campaign with a visit to Bolsover in Derbyshire.
Mr Farage attacked Labour for having "completely lost touch with ordinary people".
"They are a party completely committed to overturning what you voted for."