In 1909 the Conservatives were re-branded (as we would say today) as the Conservative and Unionist Party. It was a signal of its opposition to Irish Home Rule. To some it still matters. The Prime Minister Theresa May told the Scottish Tories last week that maintaining the Union was her "personal priority".

Even as she spoke, even her own party is questioning the importance of the Union. Leave aside the 60 per cent of party supporters who voted for Brexit, ironically triggering the new Irish border crisis that will unwind as the Unionist parties lose their Stormont majority. Even her English activists are no longer sure that the Union is worth the effort.

A survey of Tory activists, conducted with the website ConservativeHome, showed that three-quarters of English activists believe that devolution has been harmful for England. Most would prefer to maintain the Union, but nearly a third think break up would bring an "end to unreasonable demands" on England to provide ever greater financial and political concessions. That’s slightly more than those who believe the loss of Scotland would actually do serious harm to the rest of the UK.

Should another referendum campaign be launched, these Conservatives don’t want any repeat of the 2014 "vow" promising new powers to the Scottish government. Over two-thirds rule out any further concessions, with only one in a hundred supporting the voice in foreign policy that Scotland's First Minister Nicola Sturgeon craves. Tory attitudes may have been hardening since Labour’s Devolution Act. But the 2015 General Election campaign, where Conservatives were urged to exploit English fears of the SNP, will also have shaped activist views.

May’s tough speech - not even promising the devolution of returned agricultural and fisheries powers - reflects her members’ and, probably, her own instincts. It points to a "take it or leave it" campaign, gambling that the huge risks of independence will swing the vote.

At one level, the case for the Union is straight forward. Our histories, families, economies and interests are intertwined. Unpicking the strands will be painful, exhausting and leave us smaller, less able to support each other, or to benefit from our collective talents to deliver common interests.

But unionist parties are struggling to articulate a case for the United Kingdom. The different nations are evolving their own distinct political cultures and sense of their own interests. It is easy to make the unionist case to those who already identify as British; it is much harder to include those who want their Scottish, English or Welsh identities recognised, let alone those who want a united Ireland. It is harder still to argue that the inevitable conflicts of interest between different parts of the Union, where different parties win elections, can be managed within the current constitution.

The Prime Minister’s recent speech didn’t mention England; she assumed that England wants the Union and that it is the Scots who need to be persuaded. A week earlier, London's Labour mayor Sadiq Khan had attacked those who seek to "divide us". But this also assumed that we know who "us" is, and that we agree on what is in "our" best interest. But what if that is not the case? What if a growing number of English people are also questioning whether the Union works for "us"?

National identity and national interest are coming to the fore in England, not just in other parts of the Union. The recent research revealed Tory activists who feel predominantly English are most sceptical about the Union; the "British" are much less so. It is likely that the wider electorate feel the same. English discontent at the Barnett formula and Scottish MP voting rights was been clear for some years, but it may now be expressed more sharply.

For English Unionists, a wholehearted defence of the current Union risks being on the wrong side of "English" voters, just as the Remain campaign was last year. (In the Brexit referendum a huge majority of those who feel "English" voted leave; the "British" voted to remain.) May probably has enough latitude with her voters to run that risk, so long as she is not seen to offer new concessions. Labour is in more difficulty. It needs to regain support from the very voters who are most likely to feel English and most sceptical about whether the current Union is fair to them. Uncritical defence of the current constitution could create another fault line between Labour and that key part of the electorate.

A defence of the Union based on appeals to a greater good overcoming the interests of the member nations is ultimately doomed. If there is a future at all, it will means nations coming together in a reformed Union, recognising the rights and interests of each. But that will mean talking about England, and English rights and interests, as well as those of Scotland.

Prof John Denham is a former Labour MP and Cabinet Minister and is the Director of the Centre for English Identity and Politics at the University of Winchester.