TACTICAL voting.

It used to be the political pitch that dared not speak its name. In elections of yore, politicians instinctively shied away from suggesting we leave our principles at the door of the polling station. It smacked of desperation to say to people: "We know you don't like us, but vote for us anyway, because you like the other lot less."

In this election, the taboo has been well and truly broken.

It began with Lord Ashdown, who sat down at the Lib Dem conference in Aberdeen a few weeks ago, offered a memorably pithy review of Alex Salmond's campaign diaries and launched into an undisguised appeal for Conservatives mainly but anyone was likely to put pencil to ballot paper on May 7 to vote for his party to keep the SNP out.

Since then, tactical voting has become all the rage. The war of words between Liberal and Tory in parts of Scotland has become an unmissable sub-plot in this election. Polling experts have argued over the possible impact of tactical voting on the outcome. It could save Labour 13 seats, Peter Kellner, the president of YouGov has predicted. Perhaps just four, felt Professor John Curtice, of Strathclyde University. A tactical voting campaign group and been set up in Perthshire. It seems to be contagious. Sir Malcolm Rifkind, the Tory grandee, said Conservatives should "think very hard" about doing the unthinkable: backing Labour in some seats. Lord Tebbit agreed if it meant limiting SNP gains.

For Willie Rennie, the Scots Lib Dem leader, it seems tactical voting has begun to haunt his dreams.

Here is his favourite story from the campaign trail so far. "I met a lady in Crammond," he says.

"She said to me, 'I hate you. I hate your leader. I hate your party. You've done nothing in the coalition. I love Ruth Davidson. I love the Conservatives, I've always voted for them. But I'm going to have to vote for you to stop the SNP'.

"If we can get people like that voting for us, anything is possible."

Such unabashed talk of tactical voting is a sign of how the country has split down the middle since the referendum.

It is also an unsubtle commentary on the Lib Dems's whole campaign.

Crammond lies in Edinburgh West, one of 11 seats Mr Rennie's party is defending in Scotland. The polls suggest they will save just one, Orkney and Shetland, the seat held by Scottish Secretary Alistair Carmichael at the last election. The rest are being fought as a series of by-election where it is hoped tactical voting will help save well-known incumbents like Charles Kennedy and Danny Alexander and keep out an even better known challenger, Alex Salmond, who is contesting the Lib Dem stronghold of Gordon.

Mr Rennie shrugs. "We're not fans of this voting system.

"We'd like to get rid of it. But as long as we've got this voting system people need to act smartly, use their vote wisely.

"I'm always going to urge people to vote Lib Dem, of course I am. "But there is also another factor, that people will think smartly at a local level to see how best they can use their vote.

"I know in the 11 Liberal Democrat seats it is clearly a choice between us and the SNP.

"We've got lots of people from across the political spectrum and of no political opinion just saying, even though they have not voted for us before, even though they don't agree with everything we say, we want to vote for you because you are the best choice between the two options we have."

Mr Rennie, personable as ever and upbeat despite support for his party running at a paltry four per cent, speaks passionately about the need for "good liberal voices" in a parliament where the issue of civil liberties will come to the fore.

Reviews of anti-terrorism legislation and the "snooper's charter," giving new powers to the security service, are up for review. The Scots Lib Dem leader fears the SNP's march on Westminster. Based on their record at Holyrood, particularly on justice issues, he concludes the Nationalists "tend to be quite illiberal at times".

But this is hardly setting the campaign alight. Not when he is accusing the Scots Tories of failing in their patriotic duty to sit the election out.

"The Conservatives are playing a very odd game just now," he says, accusing Cameron of playing into the hands of the Nationalists since setting out his plans for English Votes for English Laws immediately after the referendum.

Now, he says, they are actively helping the SNP.

"We now see in many seats across Scotland the Conservatives pretending they can win.

"The danger in each is they will let in another Nationalist MP and that will accelerate the move to another referendum.

"They say they are the strongest advocates of the Union, and we stood together to defend the Union, but now they seem to be doing everything they can to put their party first before the country.

"Of course, everyone can argue for votes for their own party but when they do it in such as partisan and divisive way for the country, that's deeply regrettable."

He insists the Scottish Lib Dems can hold their 11 seats, despite the polls.

Across the UK, he says the party should be preferred to the SNP or Ukip as potential coalition partners or allies but he has no personal preference as to whether his party backed Labour or the Conservatives.

He cannot see the Lib Dems joining any alliance with the SNP at Westminster, however.

"If it included the SNP I don't think we could really do that.

"It is not really reasonable to put a party that is against the United Kingdom in charge of the United Kingdom."

But he adds: "It's not that we are against working with the SNP at other levels," and keeps the door open to a possible deal with Nicola Sturgeon after next year's Holyrood election, if the SNP falls short of a majority.

He says:"We could work with them. We'd have to be pretty clear about ruling out a referendum but other issues we could have discussions on."

Despite disagreements on a range of issues, he respects Ms Sturgeon's pragmatism. He adds pointedly: "It can't get worse than Alex Salmond."

Mr Rennie declines to elaborate."I'm Mr Positive this morning," he says.