A meeting of the Church of England's parliamentary body is underway in London to decide whether to allow women into the episcopate.
A majority of Church members support the move, as well as the outgoing Archbishop of Canterbury, Dr Rowan Williams, and his successor, the Right Reverend Justin Welby.
The Church of England has allowed women to be ordained as priests for the last 20 years but there is strong opposition to women bishops.
Dr Lindsay Newcombe, vice chair of Forward in Faith group, has led a prayer vigil in the run-up to Synod for opponents of women bishops.
Over 300 Evangelical and Catholic Anglican priests wrote to The Times of London last week asking Synod not to approve the measure as they felt it did not provide adequate provision for traditionalists and would do more harm than good in the long-term mission of the Church.
Signatories of the letter included Reform chairman, the Reverend Prebendary Rod Thomas, and chairman of the Catholic group in General Synod, the Reverend Simon Killwick.
"The provision being made for us in the draft Measure comes nowhere near what we need," they said.
Mr Thomas said the letter was in no way a challenge to Archbishop-elect Justin Welby.
However, he added: "His considerable mediation skills will be much needed if this Measure is not approved.
"Both evangelical and catholic Anglicans are ready to engage in immediate positive dialogue for the good of the whole church."
The legislation contains a provision for opponents allowing traditionalist parishes to request oversight by a male priest or bishop on the grounds of theological conviction.
The provision won the approval of 29 out of the Church of England's 43 dioceses.
Over a thousand Church of England clergy signed an open letter to U.K.'s Independent on Sunday urging Synod to vote the legislation through.
"The legislation to be voted on represents enormous compromise from all sides. Those who wish to avoid the ministry of women will still be able legally to do so," the letter read.
"We hope and pray that all will feel able to work together in the future with the trust and respect that should characterize our church."
Signatories included the Bishops of Salisbury, Sherborne and Ramsbury.
Church Mouse, the popular anonymous blogger who launched the Yes 2 Women Bishops initiative, said the vote "will be close".
Yes 2 Women Bishops has attracted widespread support from ordinary church members who have had little say in the legislation drafting process.
"Chatting about the issues on Twitter, it was clear that many people wanted to get involved. A few tweets later, and we had a campaign mobilized. I was inundated with offers of support to help get the word out. A few days later and we had engaged hundreds of supporters to spread the word," said Church Mouse.
General Synod member Rebecca Swinson supports women bishops and has been helping the campaign.
She said: "We don't want to re-hash the arguments for women bishops, or to debate the merits of the legislation as presented. Those debates have already been had. We want to provide an opportunity for those who have watched the debate and concluded that they want General Synod to pass the legislation to have their voice heard."
Theologian and social media expert Vicky Beeching was one of the high profile supporters of the campaign.
She said: "This is an example of the social media 'digital revolution' in full flow; giving a voice to the people, a sense of standing together, and a direct channel to appeal to the decision makers. Our 'Web 2.0' world gives everyone the potential to have a voice.
"We need 'Church 2.0' where all those in the pews get to have a say on important matters. Hopefully, Synod members will listen to what these church members are saying."
Several Synod members made special video messages under the banner of "Enough Waiting," explaining why they felt the legislation should be approved.
The Bishop of Sheffield, the Right Reverend Steven Croft, said in his video that now was the moment for a "clear decision."
"We cannot continue with a situation where women are ordained deacons and priests but not as bishops," he said.
"Of course it's been important that we take time and care to find the right ways to make provision for those who will not accept this development. I deeply respect those who hold such views. I hold them in high regard.
"They will continue to have an honoured place in the Church of England. I believe the traditions they represent have a vital part to play in our future. I will continue to support them."
The Bishop of Worcester, the Right Reverend Dr John Inge, warned that it would be a "disaster" for the Church's mission if it failed to vote for the legislation.
He said it "doesn't make any sort of theological sense" for women to be ordained as priests but not bishops.
"The legislation presently before Synod strikes what seems to me is the best balance we are ever likely to get between rejoicing in the ministry of women as bishops but at the same time honoring those who don't feel that this present move is warranted either by scripture or tradition," he said.
"It enables us to move together, respecting one another; it enables us to remain a broad church."