Lord Foulkes, who introduced a Bill in the House of Lords to stop the perk being axed, said campaigners will target the final two as they face party members in the final stages of the contest.
The peer said limiting the benefit to people who claim Pension Credit will would be “cruel”.
And he praised the Daily Express for campaigning to overturn the decision.
He said: “I have been very impressed by what the Express has been doing on this issue.”
Lord Foulkes, who worked for Age Concern when he was younger and is now the chairman of Age Scotland, said campaigners were drawing up plans to flood Conservative leadership hustings so they can challenge the candidates on the issue.
“What we want to do is get pensioners to go along and try to get a pledge from them,” he said.
The Labour peer introduced a Private Members Bill that would force the government to take back responsibility for the benefit after hiving it off to the BBC.
Corporation bosses say the scheme is too expensive and from next June will only give free licences to over-75s who claim Pension Credit.
But around 1.2 million pensioners who are eligible for the benefit do not receive it, with some unaware they are entitled to it while others do not want to admit they need extra help.
The peer said means-testing for the free licence would put off many people from applying.
“It’s demeaning,” he said. “A lot of old people are proud and don’t want to go through a means test.”
Lord Foulkes said the responsibility should shift back to the government because it is a social welfare issue.
“For so many lonely old people it’s the only form of companionship they have,” he added.
“It’s their window on the outside world. For people on the basic pension it is huge amount to pay.
“It has been a Godsend and to take it away would be really cruel.”
Lord Foulkes said the changes would hit pensioners who just failed to be eligible for pension credits hardest.
Critics of pensioner perks say the older generation are relatively better off than they were 20 years ago when the scheme was introduced while young workers are struggling to make ends meet.
But the peer said it was unfair to punish the poorest pensioners.
“It is not right to take help from poor pensioners,” he said.
Lord Foulkes said that peers from all parties had been supportive of his plans but acknowledged it was unlikely to make it through the Parliamentary process because it does not have government support.
“It is a way to put pressure on the government,” he said.