The Isle of Wight's Conservative candidate has denied claims he covered up a medical episode last year.
UKIP candidate Iain McKie said he had been told of a "major health issue" suffered by Andrew Turner, after which he was advised against driving.
Mr Turner, who suffered a stroke in 2006, admitted a stay in hospital after feeling unwell, but insisted there were "no lasting effects".
Opponents from other parties said Mr Turner's health was not an issue.
But Mr McKie said: "The last five years have been difficult - the next five are likely to be tougher still. The Island needs an MP who can be a strong advocate in Westminster for its interests."
He accused the Conservatives of deciding to "keep the details of [Mr Turner's] health difficulties a secret until after the General Election."
In a written statement, Mr Turner insisted he had been "transparent" over his health.
He has suffered from aphasia since his 2006 stroke, and said: "I felt unwell one hot day last summer, and was advised to go to hospital for a check up which I did and stayed overnight for observation as I was alone in London.
"No problems were diagnosed and there were no lasting effects; the doctor put it down to the heat.
"One is more sensitive to feeling unwell after a major incident like a stroke as many other constituents will recognise."
Mr Turner, who survived a no confidence vote in January, said he resumed driving after "tests confirmed that there was nothing to worry about" and criticised his opponent for a "smear" over his health.
The BBC has seen a report from March by outgoing Conservative Association chairman Alan Wells in which he questioned Mr Turner's ability to continue.
Mr Wells, who has previously expressed doubts over Mr Turner's candidacy, said: "I am not at all convinced it is even in his best interests to embark both on a gruelling election campaign and then enter into a further term at Westminster during which his health and reputation will surely be under intense pressure."
Other candidates for the Isle of Wight seat dismissed talk of Mr Turner's health as a distraction.
Vix Lowthion, the Green Party candidate, said: "I wish that we could just get on and debate the issues."
Liberal Democrat hopeful David Goodall said: "I believe politics is about only one thing; how to help people live happier and more fulfilling lives. It's not about personal attacks on a candidate's health."
Stewart Blackmore for Labour said: "I know Andrew Turner's had his problems but my focus is on questioning his policies rather than his health."
Independent Ian Stephens said: "It's Andrew Turner's decision to stand and his team's willingness to back him. I'm fighting on my own ability to do the job."