A rare rift between the national movements in Scotland and Wales occurred just after the devolution referendums in 1997, when the future SNP MSP Dorothy Grace-Elder provoked irritation by snidely suggesting that Wales would never have been brave enough to vote Yes by the narrowest of margins if Scotland hadn't shown the way a week earlier. There was never the slightest evidence to support that claim - in fact, it was striking how little effect the enormous Yes-Yes vote in Scotland had on the latter stages of the campaign in Wales.
However, in the case of the current general election campaign, I think it's probably fair to say that the SNP surge has had an important indirect effect on Wales. The assumption is that Leanne Wood was only invited to the UK-wide leaders' debates because it was impossible to exclude the SNP in the current circumstances, and if the SNP were there, logically Plaid Cymru had to be there as well. We're now seeing clear signs that a fairer level of coverage has weaved its magic for Plaid, which has overtaken UKIP in the latest Wales-wide YouGov poll -
Welsh voting intentions for the May 2015 UK general election (YouGov, 28th-30th April) :
Labour 39% (-1)
Conservatives 26% (n/c)
Plaid Cymru 13% (+1)
UKIP 12% (-1)
Liberal Democrats 6% (n/c)
Greens 3% (-1)
Although the changes may look like meaningless margin of error stuff, it has to be borne in mind that Plaid are up from 9% in the last-but-one poll, and that 13% is a higher than usual figure. It seems likely that Wood's involvement in the debates and her relentless focus on Wales while she was there is a big part of the explanation for this mini-surge, because a supplementary question finds that 29% identify her as the leader that best stands up for Wales, a full 17% ahead of her nearest challenger (Carwyn Jones, the Labour First Minister).
Unlike the SNP, Plaid's support is particularly concentrated in certain areas, which makes the result of the following question highly significant -
Thinking specifically about your own constituency and the candidates who are likely to stand there, which party’s candidate do you think you will vote for in your own constituency at the next general election?
Plaid Cymru 15%
Liberal Democrats 7%
It looks as if Plaid are only shedding 3% of their potential voters due to tactical/local considerations, which is being more than offset by the much larger number of tactical votes in their favour from supporters of other parties. These votes will presumably be, for the most part, in the handful of constituencies Plaid have a serious chance in. There's a maximum of six they could win - Arfon (Plaid-held), Carmarthen East & Dinefwr (Plaid-held), Dwyfor Meirionnydd (Plaid-held), Ynys Môn (Labour-held), Llanelli (Labour-held), and Ceredigion (Lib Dem-held). If they were to win five or all six, they could make up more than 10% of the informal 'progressive bloc' in the new House of Commons.