MPs will gather at Westminster Monday to resume their marathon debate on the Brexit bill ahead of the vote early Tuesday evening.
Insisting that he had not given up hope of victory in next week's vote, he said: "What is important is for MPs on all sides, Brexiters and Remainers, whatever our disagreements, to say "We are democrats and the most important thing now is to make sure that we really do deliver Brexit".
"Doing so would be a catastrophic and unforgivable breach of trust in our democracy".
"The prime minister's got to stop threatening parliament and indeed, threatening the whole of the United Kingdom, that it's a choice between her deal and no deal - that's not the case".
MPs are due to vote on May's deal on Tuesday.
Mr Blackford said that revoking Article 50, which sets a deadline of 29 March for the date of departure, and holding another referendum would be the most sensible option for moving forward.
"It will open the door to extremist populist political forces in this country of the kind we see in other countries in Europe", he added.
He predicted Mrs May was still facing defeat, but said: 'I think I decided that she needs to come within a reasonable amount of support in the House in order to persuade our Brussels colleagues that they have got to give us some clarification on the timetable of leaving the backstop.
'Imagine if an anti-devolution House of Commons had said to the people of Scotland or Wales that despite voting in favour of a devolved legislature, Parliament knew better and would overrule them.
Asked later whether Labour would push for a second referendum if there was no general election, Corbyn said: "We're then into that consideration at that point".
Brexit minister Stephen Barclay told BBC TV that persuading enough lawmakers to support the deal would be "challenging" but that even if it was rejected, he suspected parliament would ultimately support something "along the lines of this deal".
In the event Mrs May's deal is defeated, the European Union is reportedly considering extending Article 50 - the process that triggered Brexit - in order to buy more time to make amendments to the agreement.
Conservative and opposition MPs have slammed a top minister for fear-mongering after he said derailing or stopping the government's Brexit plan would see a rise in far-right extremism.
It was not possible for the minority Tory administration to control what happened in parliament, and Speaker John Bercow had shown that he was "willing to frustrate the government at every opportunity", he said.
If a confidence vote failed, he'd be under pressure to back a second Brexit referendum, which would open deep divisions in the Labour Party, many of whose supporters backed leaving the European Union in 2016.