Mark Devenport BBC Northern Ireland political editor

Not so long ago, Irish republicans were firing mortars at Downing Street. So it's a testimony to the extraordinary evolution of Northern Ireland politics in recent decades that in mid-October, Irish nationalists were expressing anger because they were not invited inside Number Ten.

It's even more extraordinary that it was the SDLP, not Sinn Fein, who were left out in the cold.

Back in the 1980s, when Margaret Thatcher signed the Anglo-Irish Agreement, there would have been no question about it.

John Hume, the figurehead of constitutional nationalism, would have been walking the corridors of power. Gerry Adams, with his IRA links, would have been deliberately excluded.

But after years of peace process politics, things have turned full circle. On Monday 13 October, Gerry Adams led his negotiators inside Number Ten. The SDLP were nowhere to be seen, and their leader did not hide his anger.

By encouraging Irish republicans to turn from violence to politics, SDLP leader John Hume courted the danger of sacrificing his party for his country

With assembly elections in the offing, it seemed a bitter blow for the biggest nationalist party in the outgoing assembly to be deemed surplus to the British and Irish Governments' requirements.

For the SDLP, the symbolism of Downing Street came after a worrying slide in the party's fortunes. In the 1998 assembly elections, the SDLP emerged with 22% of the vote - more than any other party.

They had 24 assembly seats, compared to Sinn Fein's 18. The party had capitalised on the central role played by their elder statesmen, John Hume and Seamus Mallon, in laying the ground work for and negotiating the fine detail of the Good Friday Agreement.

Republican counterparts

Mr Mallon went on to serve as deputy first minister alongside the Ulster Unionist leader, David Trimble.

However, by encouraging Irish republicans to turn from violence to politics, the SDLP leader, John Hume, courted the danger of sacrificing his party for his country.

Every time the IRA took another initiative, Sinn Fein hogged the international headlines.

As the SDLP veterans Hume and Mallon appeared to be preparing to exit stage left, their republican counterparts seemed to grow in stature.

This will be Mr Durkan's first campaign at the helm

In the 2001 Westminster election, Sinn Fein emerged with four seats - one ahead of the SDLP. Moreover, unlike the SDLP, Sinn Fein built electoral bridge heads on either side of the border, with five deputies in Leinster House in Dublin.

The SDLP will have their work cut out holding back Sinn Fein's challenge in these assembly elections. None of the SDLP's three Westminster MPs are standing as candidates.

Far from being young upstart extremists, Sinn Fein's Adams-McGuinness leadership looks more experienced and assured than John Hume's successor, Mark Durkan. This will be Mr Durkan's first campaign at the helm of the SDLP.

As memories of the Troubles fade, young nationalists don't have the kind of moral problems voting for Sinn Fein which their parents might have experienced.

If you are a first time voter, you will probably think of Martin McGuinness as the former local education minister - not the former second in command of the IRA in Derry.

'Silver lining'

Nevertheless, the SDLP should benefit from the system of PR voting. The party is more likely to pick up transfers than its opponents in Sinn Fein. So even if a Sinn Fein candidate comes out ahead on the first round, their SDLP counterpart may overtake them when transfers are taken into account.

Moreover, the SDLP's exclusion from the Downing Street talks now appears to be a two edged sword. After all, the talks did not produce a deal.

Without saying "I told you so", Mark Durkan is able to argue that it requires the involvement of more than what he dubs the two "problem parties" (the Ulster Unionists and Sinn Fein) to make the Good Friday Agreement stick. For the SDLP, the continuing cloud over the peace process has a silver lining.

Gerry Adams led his negotiators inside Number Ten.

Policing should prove a key battle ground for the nationalists. The SDLP has already joined Northern Ireland's Policing Board and is encouraging young nationalists to join the new police service.

Many SDLP activists have suffered intimidation and violence because of their membership of District Policing Partnerships. Sinn Fein is still holding back, although the emphasis that Sinn Fein negotiators placed on an early devolution of policing and justice powers makes it seem like only a matter of time until republicans follow suit.

Will voters reward the SDLP for setting a brave example? Or will they support Sinn Fein's attempt to barter for more policing changes?

This time the SDLP will argue that a vote for them is the safest bet to sustain the Agreement. Sinn Fein might counter that if unionists are likely to return a more hardline team of negotiators, then nationalists should follow suit.

Some talked about the last Westminster election in West Tyrone as the SDLP's Stalingrad, but right across Northern Ireland this poll will test the party's mettle as never before.