A vibrant democracy needs a noisy, edgy, vibrant civil society." These were Acevo chief executive Stephen Bubb's words to the public administration committee last week, during an evidence session of its inquiry into charity regulation. Bubb was moved to defend the right of charities to campaign after Charlie Elphicke MP suggested that political activity was damaging public confidence in the sector.

Elphicke had two recent cases in mind. In September there was Save the Children's report into child poverty in Britain. Its headline finding – that one in eight of the UK's poorest children go without at least one hot meal a day – was quickly picked up by Labour, who blamed the shocking statistic on the government's "chaotic decisions". When the right-leaning media twigged that Save the Children's chief executive was a former Labour adviser, Conservative MPs lined up to denounce its "political agenda" .

Soon after, the Telegraph reported that a group of Conservative MPs – including home secretary Theresa May – had joined the Fawcett Society enmasse in an attempt to stop the gender equality charity acting as a "mouth piece for Labour". They claimed that the society's campaigning – including a legal challenge to the 2010 emergency budget – was at odds with its charitable objectives.

There may be a strong element of mischief-making in both these cases, but Elphicke could be right about public distaste for overtly political campaigning. An Ipsos MORI poll in June found that 7% of those whose trust in the sector had decreased blamed "political bias/pressure". Not a huge figure perhaps, but it is more than three times what it was in 2010 – before high profile campaigns against welfare reform and spending cuts.

At first glance, the rules on political activity are clear. Charities have every right to campaign for a change in law or policy if it would help achieve their charitable purposes. The important caveats are that this cannot become a charity's sole purpose and that explicitly party political activity is forbidden. The case of Atlantic Bridge seems an obvious case of a charity crossing these lines.

But the length of the commission's guidance on the issue – almost 14,000 words – suggests there are some expansive grey areas. What's striking is the emphasis it places on public perception; it is not enough for a charity to act impartially, it should also "stress and make clear its independence".

What does that mean in practice? The commission advises charities to ensure the language they use is politically neutral, to ensure speaking invitations are extended to all parties and to use MPs' photographs judiciously. A commission spokesperson even suggests including an impartiality disclaimer on campaign materials.

But demonstrating political independence isn't just about keeping the regulators happy – it usually makes for more effective campaigning too. "We would always advise charities not to put all their eggs in one basket, politically speaking," says NCVO's parliamentary and media manager Chloe Stables, "and to work across the political spectrum building consensus because it's a much better way of getting public policy change through."

It's a view shared by Estelle McCartney, a director of public affairs agency Champollion, who says charities that focus their campaigning on too narrow a part of the political spectrum risk harming their reputation. "In the short term they can damage relationships with whichever political grouping that they have antagonised," she says. "In the longer term it damages their overall credibility if they're seen to be acting from a position of partisanship rather than a position of informed, evidenced policy contribution."

McCartney, who is also a trustee of IndependentAge, says there are two important principles that should help charities avoid accusations of bias. The first is ensuring that any claims made as part of the campaign are meticulously researched and backed by evidence. The second is pursuing cross-party engagement long before the campaign has been launched – attacking government policy will never be popular with government MPs, but opening up a dialogue in advance can at least ensure they are not caught off guard by a campaign run solely through the media.

It is not only politicians that campaigners need to persuade, however – often their biggest challenge is winning over the public. Linda Butcher, chief executive of campaigning support charity the Sheila McKechnie Foundation, says this is made all the more onerous when campaigns don't look beyond their natural supporter base. "Very often you look at who they're focusing on and it's people who are already behind the cause," she says. "But actually, when you have limited resources and you look at the impact you need to make, you should be spending more time on people who don't agree with you."

The sector will almost certainly face more accusations of political bias over the coming months, some of which will be disingenuous attacks on the right of charities to campaign at all. But by asking if their tone and messages could be alienating potential supporters – and if too many resources are focused on those who already agree – campaigning charities could become more effective as a result.

James Gray is a freelance writer and charity consultant.

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