The coalition's controversial gay marriage legislation returns to the Commons with Tory MPs and activists deeply split over the issue.
The move has been championed by David Cameron but he has faced Conservative opposition at all levels from the grassroots to the Cabinet .
A group of more than 30 current and former local party chairmen warned that the plans would drive Tory voters to the UK Independence Party and make a Conservative election victory in 2015 impossible.
Bob Woollard, chairman of the Conservative Grassroots group which organised the protest letter to the Prime Minister said: "Same sex marriage is really a tipping point, a bellwether issue if you like - people have just said 'I've had enough, I'm off, I will never vote Conservative again'.
"Scores and scores and scores of people that we have all spoken to, probably hundreds of thousands of people have said: 'I've had enough, that's it now, we can't cope with this so-called modernisation agenda. We are not voting Conservative again until this bill is scrapped, defeated in the House of Lords, kicked into the long grass or until there's a change of leadership'."
But a rival letter, signed by more than 100 Tory activists, called for Conservative MPs to "deal with the Bill then move on together as a party".
The Marriage (Same Sex Couples) Bill will be debated over two days, with its third reading - the final hurdle in the Commons - tomorrow.
Tory former minister Tim Loughton will attempt to amend the legislation to allow heterosexuals to have the same right to civil partnerships as gay couples.
He highlighted a ComRes survey of 159 MPs from across the parties which found 73% agreed that civil partnerships should be extended to heterosexuals "in the interests of equality" if gay marriage is legalised.
Mr Loughton said: "Far from being a 'wrecking measure' some of the strongest support for my amendment to extend civil partnerships comes from the biggest supporters of same sex marriage in the Labour and Lib Dem parties."