A watchdog insisted it was acting "with complete impartiality" after the Conservatives raised concerns about potential bias in an investigation of the party's election expenses.
Tories face an Electoral Commission probe and police inquiries over allegations that spending on buses transporting activists to campaign in key marginal seats was not properly recorded.
Channel 4 News said the party's lawyers had written to the Commission about anti-Tory social media posts by the woman who is now its head of compliance.
Louise Edwards was said to have written after the 2010 general election that she did not "want to live under a Tory government" and "can't understand what people were thinking - do they not remember the Tories before?"
The Commission said the posts were from five years before she joined the watchdog and "do not impact" on the investigation.
A spokesman said: "We do not comment on ongoing investigations.
"However, we are satisfied that the Commission is acting fully in accordance with our enforcement policy and with complete impartiality.
"The Commission is very mindful of the need to ensure that it acts at all times impartially.
"Extracts from an employee's Facebook profile that have been reported in the media were taken five years prior to that employee joining the Commission and do not impact on the investigation currently being undertaken by the Commission."
The dispute emerged as a senior Tory MP also wrote to the elections watchdog to highlight the spending of rival parties on their battle bus campaigns.
Charles Walker urged it to "clarify its guidance" about the rules governing campaign buses and also highlighted SNP leader Nicola Sturgeon's use of a helicopter to travel to battleground seats.
He said that if the commission failed to examine the Labour, Liberal Democrat and SNP campaigns it could give the impression it was "behaving in a way that could lead to it being accused of political bias".
The Conservatives are being investigated over allegations that the cost of its battle bus 2015 campaign should have been recorded as counting towards individual candidates' spending limits, rather than as part of the larger national return.
Broxbourne MP Mr Walker wrote to Electoral Commission chairwoman Jenny Watson saying "a number of political parties have interpreted the rules on election spending in a similar fashion to the Conservative Party".