"HELL, yes - I'm tough enough." It has become the catch phrase of the general election, recycled by commentators, comedians and the Twitterati meme-merchants.

Ed Miliband's response to aggressive questioning from Jeremy Paxman about his ability to take the political heat has certainly done his image no harm. I don't know about the politics but it was great television.

Sky's "battle for number 10" was old-fashioned national event television - with a digital twist. Its huge viewing figures were dwarfed by the activity on social media. Twitter was registering 2,700 comments an hour. Politics has become a mass participation sport. These debates are like cup finals where everyone can be in the commentary box.

Watching it, I initially felt as if I had been time-warped back to the spin room in the great debates during the Scottish referendum campaign. Alistair Darling's former spin-doctor Catherine Macleod even popped up on Sky's preview programme as a pundit.

There was that same sense of the campaign being rebooted by a brutal contest where nothing new was learned about the issues but where voters got to see what the contenders were made of.

It now seems pusillanimous of Cameron to have refused a head-to-head debate with Miliband, along the lines of the referendum debates. It made him look like the one who's frit.

These debates are a great opportunity to change fortunes. Most voters had formed a pretty negative opinion of Ed Miliband because of pictures of bacon sandwiches and the panda eyes. Now people were seeing him as if for the first time.

And while the Labour leader didn't look or sound as fluent as the PM, or as comfortable in his own skin, Ed certainly made an impact - which is what he had to do. He won the social media battle, scoring a victory in positive tweets, even though the snap opinion polls gave the night to Cameron.

Which has consequences. For the first time in months, the SNP's commanding lead in Scotland looks vulnerable. Not because of the opinion polls, which continue to register Nicola Sturgeon's extraordinary lead over Labour. Not because the issues raised - immigration, deficit, Europe - were particularly damaging to the SNP (indeed Scotland hardly figured). But because of a structural bias in the UK media which focuses attention on the leaders of the UK parties.

Now, I'm not accusing Sky, the BBC or anyone else of political bias, let me be clear. But it is in the nature of Westminster elections that they focus attention on the UK leaders. The SNP is always a side player in general elections because it doesn't stand across the vast majority of the UK and can't be in contention to lead government. This is just a fact of political life.

Consequently, UK general election campaigns tend to take the appearance of presidential contests of personalities between the leaders of the two main parties. And these leader debates are becoming the key moments in which voters begin to form their attitudes. There is no way this can fail to bleed over into Scottish politics.

Look how last week's exchanges spawned follow-up debates about the specific issues mentioned, such as immigration, zero-hours contracts, food banks, deficit reduction. The debates cast a large shadow over the media of the entire country for the next week - whether on Facebook or in the newspapers.

It's not clear how Nicola Sturgeon can fight this marginalisation. One of Alex Salmond's great gifts was his ability to cut through it all and make news almost wherever he goes. True, he often makes news for the wrong reasons and has an irrepressible gift for confrontation. But no-one can ignore him.

Recall his exchanges with Anna Soubry on the Marr Show. Or his pink champagne lunch with the New Statesman. Labour suggested that this drink, which is now a downmarket staple of hen nights, somehow indicated elitist tendencies. But what it really indicated was that Salmond can't order a drink without it becoming news.

Nicola Sturgeon is a very different kettle of fish, if you'll excuse the pun. She doesn't do confrontation, isn't always ready with a clever sound-bite, and doesn't tend to hog media attention. She is immensely popular in Scotland, where she needs to be, but her impact on the UK scene has been minimal, even though opinion polls suggest she is not unpopular. It's just that no-one really knows her.

This may be a good thing, and her advisers claim it is. Salmond was a controversial "Marmite" leader, who generated a lot of hostility, not least in the press. Nicola Sturgeon seems to have a very much wider appeal and as she showed at the SNP conference, she is very much in charge of her party.

Focus groups suggest that she hits home with the people Salmond never could: women and middle-class voters. She doesn't feel overshadowed by the former leader, but she is going to have to work hard to make an impact in this campaign, dominated as it is by the metropolitan media.

There has been much speculation that Sturgeon resents Salmond's star quality, prominence and his tendency to make policy on the hoof. But she doesn't particularly mind Salmond doing this so long as they are the right policies.

Things will change soon after Salmond completes his book tour and retreats to the sidelines - that's if Alex is capable of finding the sidelines. But she's the one with the job, not the one with the gob.

Sturgeon will of course be appearing in the "magnificent seven" debate of all the party leaders. And she will have ample opportunity to appear in television debates in Scotland. But she is not a Twitter meme and probably never will be.

What Sturgeon does bring to the table is distinctive policies: not just opposition to austerity and renewal of Trident. She is pro-Europe in a way Ed Miliband cannot be. She can talk freely about a living wage, child care and open immigration.

This gives her a distinctive place in the national conversation - not least because she could be instrumental in deciding the next government. The recent opinion polls suggest that neither Labour nor the Tories will get an overall majority and will have to seek a coalition. Labour spokespeople in Scotland are trying to suggest that Ed Miliband would walk away from government rather than rely on the SNP to get his Queen's Speech through Parliament. Jim Murphy has suggested he might not serve.

This would be an abdication of democratic responsibility and constitutional duty. If a party leader can get his legislative programme approved by the Commons without a formal coalition, he has no right to walk away. The notion is anyway preposterous.

If Labour did this, and allowed the Tories back in, it would destroy the party forever in Scotland. And anyway, the SNP could still vote down a Tory Queen's Speech if the polls remain as they are.

Hell, yes - Ed needs the courage to ignore the nonsense about being in Salmond's pocket or under Sturgeon's heels, and to seize the moment. Is he tough enough?