Campaigners trying to ensure grass roots Conservative members keep the final say on choosing a new leader claim they are on course to succeed.
Mr Howard is expected to step down in October
The Better Choice campaign has also not ruled out the possibility of a legal challenge to the attempt to change the rules for the leadership contest.
A ballot on the rule change is due to close on 27 September.
A leading member of the Conservative ruling board said the rule change process had been handled correctly.
Under the current rules, Tory MPs whittle down the field to two candidates to go to a vote of all party members.
The system has only been used once - to elect Iain Duncan Smith - and critics say it results in a leader who cannot necessarily command the support of MPs.
A constitutional college of MPs, MEPs, peers and party activists is voting by post on a proposed amendment which would give MPs the final say on the new leader, with grass roots members only consulted.
There has to be a two-thirds majority among activists and MPs to change the rules.
The Better Choice group says it has enough votes among both groups to block the plans.
But it argues the ballot is not valid because, they say, it was not triggered by a formal resolution of the Conservative Party board.
It is understood the group's lawyers believe there are solid grounds for a legal challenge.
But with the group saying it is very confident of the rule change being blocked, it has to decide whether such a challenge would further its cause.
Raymond Monbiot, chairman of the Tory national convention and a member of the party's board defended the ballot.
"There was a formal resolution of the board on 4 August," he told the BBC News website. "It was done absolutely correctly."
Mr Monbiot said 400 of the 830 ballot papers for national convention members had already been returned.
He insisted the meeting of local Tory chairmen to discuss the plans on Saturday was "very healthy", with people speaking for and against the rule change plan.
"It is probably the most vibrant discussion that we have had for years," he added.
David Davis, the bookies' favourite for the leadership, says he thinks the rule change plan will be defeated.
Rival Ken Clarke has urged caution over such speculation. He also denies he would lose out if the vote rests with party members, who preferred Mr Duncan Smith to him in 2001.
Other potential candidates include Sir Malcolm Rifkind, David Cameron and Liam Fox.