Left-winger John McDonnell still needs the support of 18 more MPs if he is to challenge Gordon Brown for the Labour leadership, the party has revealed.
Mr McDonnell said Labour members deserved a choice of leader
So far, 27 MPs have nominated him, compared with 282 for the chancellor.
In the deputy leadership contest, Hilary Benn has yet to gain the 45 nominations needed.
Jon Cruddas, Alan Johnson, Harriet Harman, Peter Hain and Hazel Blears have all got through. Nominations are due to close at 1230 BST Thursday.
Mr McDonnell said: "This is really good progress. It's not even half-time yet and we're over half-way in terms of nominations with some MPs out of the country and others finalising their position.
DEPUTY CONTEST SUPPORT
Hilary Benn - 34 MPs
Hazel Blears - 48 MPs
Jon Cruddas - 46 MPs
Peter Hain - 49 MPs
Harriet Harman - 60 MPs
Alan Johnson - 64 MPs
"This confirms that we're well on course for getting on the ballot paper and with this momentum we should make it."
Among his supporters were backbenchers Diane Abbott, Jeremy Corbyn, Dennis Skinner, Ian Gibson, Ann Cryer, Mike Wood and former rival for the leadership nomination Michael Meacher.
Tony Blair has nominated Mr Brown, as have Home Secretary John Reid and Pensions Secretary John Hutton.
But former Blairite Cabinet ministers Charles Clarke, Alan Milburn and Stephen Byers - who have all said they will support for the chancellor in a demonstration of unity - have yet to nominate him.
Earlier, Mr Johnson, who is backing Mr Brown for the leadership, said he thought Mr McDonnell was "unlikely" to get enough nominations.
Mr McDonnell is the chancellor's only challenger after rival left-winger Michael Meacher dropped out on Monday, in the hope they could pool their support.
Some 43 of Labour's 352 MPs have still to nominate.
Earlier, Mr McDonnell's spokesman told the BBC News website said there would be much support among Labour's grassroots for Mr McDonnell's opposition to the Iraq war, privatisation and university tuition fees.
'David and Goliath'
"There's terrific enthusiasm for a genuine debate. If John gets on the ballot, Gordon Brown will face a serious challenge," he said.
"Obviously Gordon is the frontrunner. It's a David and Goliath situation but this is a serious challenge not just for the leadership, but the party's direction."
Mr Meacher reiterated his support for Mr McDonnell and urged his backers to switch their nominations to him.
But Mr Johnson, who is one of six candidates for the deputy leadership, said: "I think the media will focus more on the deputy contest because it looks unlikely there will be a contest for the leadership."
Later Mr Brown told BBC Radio 4's World at One programme that while he welcomed a contest, he would not be "lending" any of his supporters to Mr McDonnell.
"That would be dishonest," he said.
"I'm going out to get as many nominations as possible because I believe that is what someone who's putting their name forward for leadership should do."
Later, in a speech to the Confederation of British Industry, he outlined his vision of a Brown premiership in a speech to business leaders.
He promised to reduce innumeracy among primary school leavers with one-to-one tuition and offer more apprenticeships for teenagers at risk of dropping out of school.
The chancellor also pledged to "never take any risks with inflation or with the stability of the economy", and said the challenges of globalisation had to be met with "world class" education, skills and innovation.
There will now be 10 official hustings events before the results are announced at a special one-day leadership conference in Manchester on Sunday 24 June - three days before Tony Blair steps down as prime minister.
Deputy leadership candidates also need the support of 45 MPs to progress.