Areas such as the blindingly beautiful Lleyn Peninsula and Snowdonia may embrace nationalism, but in the heavily populated south, the party's appeal ranges from limited to almost non-existent. The nadir came on 16 May 1991 at the Monmouth by-election when the Plaid candidate with tacit Green Party support polled 277 votes to Screaming Lord Sutch's 314.
Since then, helped by the fact that Conservative support in Wales was in freefall, the nationalists have made a recovery, of sorts. They outperformed the Tories at the 1994 Euro-elections by 162,478 votes to 138,323. And in last year's local government poll, when 22 new unitary authorities covering the whole of Wales were set up, Plaid returned 113 councillors.
That's the good news. The bad news for nationalists is that in racing terms, Labour won both contests by a distance, taking all five Euro seats with 530,749 votes and romping home west of Offa's Dyke with 731 councillors.
Dependence on the language vote is made evident by the fact that Gwynedd, which includes Snowdonia and the Lleyn, is the only local authority controlled by the nationalists. The county has 45 Plaid councillors, more than a third of the all-Wales total. Such facts undermine Plaid's claim to be "the party of Wales", a soubriquet fated to remain a Celtic spin-doctor's slogan, bearing in mind the loss of 20 deposits at the last general election.
In 1992 Plaid contested all 38 seats and returned four MPs, all from constituencies on the fringe of a Wales that is itself reckoned to be on the periphery by arrogant metropolitan opinion.
Dafydd Wigley, MP for Caernarfon and party leader, remains bullish about Plaid's prospects in the poll that matters most - the looming general election. "We have hopes of winning Carmarthen East and Dinefwr from Labour and Clwyd West from the Tories," he ventures. The Clwyd seat, the safest of the Tories' decidedly shaky half dozen in Wales, is held by Rod Richards, the right-winger who earlier this year resigned as Welsh Office under- secretary following allegations of an extra-marital affair.
To the surprise of many, the blue rinses of the constituency - known as the Costa Geriatrica on account of the large number of retired people - helped Mr Richards see off a move to deselect him.
Plaid is defending a majority of 1,106 in Ynys Mon. Ceredigion promises a close four-way battle where the defending nationalist MP, Cynog Dafis, acknowledged to be Britain's greenest MP, has lost the support of the Green Party following a spat over fox-hunting.
The nationalists insist that devolution is a trump card. All parties except the Tories are committed in some shape or form to shifting power from London to Cardiff.
After a somersault, Labour, if elected, has settled for a referendum before introducing legislation.
Events are beginning to move. Earlier this week BBC Wales published the findings of an opinion poll signalling a significant shift from the four- to-one rejection of devolution in 1979. Now just over half (55 per cent) of the sample of 1,000 questioned by Beaufort Research back the creation of an elected Welsh assembly. Some 28 per cent were against change, with 17 per cent undecided. Ron Davies, the Shadow Welsh Secretary, rejoices that 83 per cent of those questioned backed a referendum.
The preferred option - an assembly with fewer powers than those proposed by Labour for a Scottish parliament - is not to Plaid's taste. Mr Wigley calls for a four-question referendum - the status quo, Labour's plan, an assembly with law-making powers and full self-government within five years.
"If all we get is a "yes" or "no" question then it's a waste of time. When we see what's on offer we'll call a special conference to decide our position," he says.
But to many people, devolution is not the most important issue. Confronting the blight of a low-wage economy, buttressing a stressed farming industry light years away from the prosperity of the grain barons of East Anglia and repairing a social fabric ripped apart by the demise of coal-mining and other heavy industries take centre stage.
Dr Denis Balsom, a political analyst at the University of Wales, puts it like this: "Devolution is important but I can't help pointing out that some of the emphasis on it is to do with anti-government feeling. Nevertheless, coming from the four-to-one defeat of 1979 is an impressive turnaround."
This morning, Alex Salmond, leader of the Scottish National Party, will address the Plaid Cymru conference. SNP support spreads more widely than Plaid's. Nationalism north of the border has twice triumphed in by-elections at Govan, a constituency as industrial as, say, the Pontypridd seat of Labour's Kim Howells. At the 1989 by-election Howells' Plaid opponent polled less than half the Labour vote. Once the SNP leader sits down today delegates are scheduled to debate prison reform. That may be an easier assignment than forming plans to break out of their own electoral confinement.