A Nationalist tsunami is sweeping through Scottish politics. It will probably overwhelm most of the Labour and Lib Dem MPs in its path. And there is a widespread assumption that it will swamp Scotland’s one Conservative MP too.

Yet David Mundell does not look like a man who expects on Thursday night to be drowned by the Nats. When I met him yesterday morning in Langholm, a quiet town in his vast constituency of Dumfriesshire, Clydesdale and Tweeddale, he seemed quite relaxed.

Mundell has been written off before. Authoritative predictions of impending defeat were made before he won this newly created seat for the Conservatives in 2005, and some obsevers assumed he would lose it in 2010, when he actually managed to increase his majority over Labour to 4,194.

So as Mundell himself put it, over a restoring cup of coffee in Pelosi’s Corner Café: “I’m used to a competitive situation. It’s just that the opposition is slightly different this time round.”

He recognises that something dramatic is going to happen on Thursday: “It really will be a tsunami, and I have to be on high enough ground. There are many Labour and Lib Dem MPs who are on the beach.” But he reckons he has a better chance than most Scottish MPs of holding off the challenge from the Nats: “They’ll have to come over the top of me.”

For his own vote is not collapsing. Mundell described it as “rock solid”, a description I have no reason to doubt after walking round Langholm with him. He was recognised by a high proportion of the people we met, and many assured him of their support. He is a man with no side, who communicates an air of good-humoured calm.

On his surgery tours, Mundell has visited about a hundred different communities, in order to give his constituents the chance to meet him. One of his canvassers said: “It’s hard to find someone in this area who doesn’t know someone David’s helped.”

Mundell had begun the day by addressing 50 volunteers in a car park in Gretna, another of the small towns in his constituency. Their aim was to conduct “a big sweep” of Conservative voters and encourage them to turn out on Thursday. So as Mundell himself conceded, they did not really know what was happening in traditionally Labour areas in the north of the constituency, which stretches for 70 miles up the M74 motorway.

The point nevertheless stands that the Conservative vote is holding up. An elderly woman with her hair in blue and green rollers told him: “All the best. I hope to goodness you get in. I’ve seen Ed Miliband with his poster.”

She recalled an occasion when she told Mundell about “all the wood that was stuck against the bridge”- Langholm stands on the River Esk – and the debris was removed within two days. He has the advantage of being a candidate with local roots: not only was he brought up here, and educated at a local school, Lockerbie Academy, but in the course of the day, it emerged that Mundell’s grandmother was postmistress in Wamphray, a village between Lockerbie and Moffat, for 36 years, from 1932 to 1968.

Another lady, who was tending her front garden with a pair of sheers, assured Mundell that among the candidates for the seat, he was “the nicest looking one of the lot”, and she had put his picture on her wall.

According to Mundell, the “biggest challenger in relation to the Conservative vote is the Grim Reaper”. In some other seats, Conservatives may decide that in order to defend the Union, and defeat the Nats, they have to vote for another party. Mundell is not vulnerable to that calculation. In this seat, he is plainly the Union’s best hope. The same can be said of John Lamont MSP, Conservative candidate in the neighbouring seat of Berwickshire, Roxburgh and Selkirk, who is seeking to defeat the incumbent Lib Dem, Michael Moore, but himself faces a strong challenge from the SNP.

The only posters I noticed while walking round Langholm were for the SNP. Mundell himself was wearing a fine blue rosette, pinned to the informal red jersey he was wearing, and his canvassers mostly wore rosettes too. But he remarked that his voters are generally “more understated”, and are not given to displaying posters, whereas part of “the Nationalist phenomenon” is that “you very much wear your politics on your sleeve”.

In 2010 the Nats came fourth in Dumfriesshire, Clydesdale and Tweeddale, with just under 5,000 votes. The question now is whether they can attract disaffected Labour and Lib Dem voters in sufficient numbers to overtake Mundell.

Murray Tosh, a former Member of the Scottish Parliament who was out canvassing for Mundell, recounted meeting “a Nationalist who’s going to vote for him – he’s worked out that the SNP is very left-wing”. Mundell himself observed: “”People aren’t going to go out and shout, ‘ I’m Labour but I’m voting Tory’.” So this phenomenon is by definition almost impossible to quantify, at least until we can look at the actual voting figures.

The redoubtable Annabel Goldie, leader of the Scottish Conservatives from 2005-11, had also arrived to canvass for Mundell. She said the success of the Scottish Conservatives under Ruth Davidson’s leadership during the referendum campaign had put them in very good heart.

After canvassing in Langholm, Mundell drove westwards across the constituency to Dumfries. To our left, sublime views opened up across the Solway Firth to the Lake District. This is a wonderful part of the world, and the idea that the Lake District might in future be part of a foreign country seemed absurd.

Mundell is convinced that the Labour voters who decide this time to back the SNP are not voting for separation. He pointed out that both Labour and the Lib Dems had long campaigned as anti-Tory parties: a role now stolen from them by Nicola Sturgeon and Alex Salmond, who can promise to cause the Tories far more trouble at Westminster than Miliband would.

On the outside of Langholm Town Hall there is a tablet in memory of William Julius Mickle, poet, born in the town on 29th September 1734, whose works include “There’s Nae Luck Aboot the Hoose”.

On Friday morning, the Scottish Tories may or may not be lamenting their luck. But it seems unlikely that the number of seats they win will reflect their share of the vote.

Mundell nevertheless rejected any idea that the first-past-the-post system of election should be changed. He said that it “establishes a connection between the MP and the constituents”, and if party lists were introduced, then in a situation like this, the Labour leadership would escape intact. As it is, “We’re going to see some pretty senior Labour figures go.”

For any Unionist, the situation in Scotland is highly alarming. But after seeing Mundell and his volunteers in action, I am inclined to think that the crisis has also had an invigorating effect, and has put fresh spirit and a fresh sense of direction into the Scottish Conservatives. Here is an unashamedly Unionist party, whose leader, Ruth Davidson, articulates better than Labour or the Lib Dems the desire of the majority of Scots for the Union to continue.