Sir Michael Spicer has been accused of beginning a "dishonest" campaign ahead of a possible General Election in a hotly contested Worcestershire constituency.
Liberal Democrats in West Worcestershire have accused their Tory rivals of duping recipients of a survey into thinking it is completely anonymous.
A code on the top corner of the survey means voters could be targeted in the run up to the vote - despite declining to give their address. The Conservatives have denied they intend to use the details.
The spat comes as Sir Michael is expected to be involved in a tight contest with Liberal Democrat district and county councillor Tom Wells if an election is held on Thursday, May 5.
"A lot of people believe they are doing it anonymously. While the wording is careful and not illegal, it is dishonest," said John Morgan, Liberal Democrat campaign organiser in the constituency.
"A lot of people are completely happy to let Sir Michael Spicer know what their views are but do not want it to be known who they are.
"Our concerns are that the electorate is unaware that by completing this survey they are identifying who they are. The Conservatives are collecting information that people don't necessarily want to give."
The survey, sent out to most voters in Malvern, asks for views on national and local issues.
Mailing list
It allows respondents to include an address to receive more information from the party or indicate they do not want to put on a mailing list.
But the Liberal Democrats say people who leave both sections blank - thinking they will not receive material because they have not provided an address - could still be identified through the code, found in millimetre-high print in the corner of the questionnaire.
Simon Eardley, Sir Michael's parliamentary agent, acknowledged the code could be used in promotions "if we were so inclined, but we do not".
"Basically, this is an opinion-gathering exercise. We respect wishes to remain anonymous," said Mr Eardley.
He said the electoral registration details acted as an internal check to help with printing and distribution.
Sir Michael declined to comment further, saying he supported Mr Eardley's explanation.
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