The election campaign is only weeks old but the intense battle to win the east Belfast seat has been raging for five years.

The powder-keg struggle between the incumbent Alliance Party and the Democratic Unionists they ousted in 2010 has reverberated well beyond the constituency and into the wider political sphere, never more so than during the febrile Union flag protests.

Sitting MP Naomi Long, who dethroned DUP leader Peter Robinson five years ago, is now facing a tough fight to hold off former DUP Belfast lord mayor Gavin Robinson - a namesake but no relation of Stormont's First Minister.

It is a challenge that has grown significantly due to manoeuvring within the wider unionist family that has removed a number of other candidates from the race.

A formal pact between the DUP and Ulster Unionists saw the latter exit the stage, while smaller unionist/loyalist parties have also declined to enter the contest.

With more than 9,150 votes cast in 2010 for pro-Union parties not taking part this time round, Mr Robinson's bid to overturn Mrs Long's 1,500 majority has undoubtedly been strengthened by the reduced field.

Queen's University academic Peter Shirlow said if it was a case of "simple arithmetic" the DUP should win by a substantial margin.

"But there is no simple arithmetic that says Protestants will vote for a pact," he said.

Professor Shirlow said it was difficult to assess how many unionists who did not vote DUP five years ago would switch support to the party this time.

"Unionists who have a strong sense of Britishness would agree with pacts, they will see pacts as vital," he said.

"But there will be other unionists, especially those who are more liberal, who will have difficulties voting for the DUP and they may shift to the Alliance party."

He added: "So as much as the pact will probably carry Robinson over the line there is still a wee bit of wriggle room for Naomi Long to work on.

"She certainly has a chance. There's no doubting she has a chance, but she needs to really bring together what is a very loosely defined coalition of liberals, unionists, nationalists and even some republicans."

Mrs Long doesn't believe the pact has made defeat inevitable.

"I've always been the underdog in every election and I have won a fair share so I am not going to focus on that," she said.

"What we are hearing on the doors is people are not liking their vote being taken for granted - the assumption that simply because the Ulster Unionists aren't standing that everyone will automatically vote for Gavin I think is a mistake.

"There will also be people out there who didn't vote the last time but will be motivated to because they don't like the pact - they don't like the idea parties are ganging up essentially to remove an MP for no good reason other than a personal agenda - and I think that's how people see it.

"I always look at it through the lens of the last election. In 2010 I was in third place at this point out, there were no pundits saying I was going to win the seat. I was 100/1 when the betting opened (in 2010) on the seat in east Belfast to win, so to start out at 5/2 this time puts me in a much stronger position than I was last time."

Mr Robinson rejects criticism of the pact and said "lofty" analysis that denounces it as sectarian or not progressive is well wide of the mark.

"This is what people have asked unionist politicians to do," he said.

"On the doors we are hearing how satisfied people are that finally this has happened, the most comprehensive agreement (between the DUP and UUP) in 30 years."

The trained barrister maintains that unionists coalescing around one candidate is not a throw-back to old politics but actually a modern response to a constitutional issue that has been reawakened by the Scottish independence referendum.

"The context of the Scottish referendum last year, it showed how precarious the Union as a whole can be," he explained.

"So if you are interested in the Union, that's not being tribal. It's not being sectarian if you see the benefits of the Union."

He added: "I wouldn't say the referendum entrenched positions (in Northern Ireland), what I would say is it's galvanised support for something that was seen as a settled issue nationally.

"The threat didn't come from Northern Ireland, the threat was from another constituent part. But it has galvanised people who recognise that actually the Union is a good thing.

"There is much to be gained from being part of a wider, political and economic union and people want to see that maintained."

The fallout from the flag protests is still inescapable in east Belfast, more than two years on from the height of the controversy.

Alliance's decision to support a SDLP/Sinn Fein move in Belfast City Council to limit the flying of the Union flag over City Hall enraged unionists and loyalists.

While this sparked heated rows between Alliance and its unionist rivals in the political arena, there was also a more sinister element, with Alliance offices attacked and party representatives, including Mrs Long, receiving death threats from hard-line loyalists.

Professor Shirlow highlighted two potential consequences for the general election.

"Very few Alliance votes will stay with them in working class areas so she has to carve out a vote somewhere else," he said.

But he added: "I think a thing that might help Naomi Long is the fact the Alliance party during the flag protests came under a great deal of physical and emotional challenge.

"There could be a part where liberal, Protestant women might think 'Naomi deserves our vote because of what she has been put through'."

Mrs Long characterises the campaign as the most antagonistic she has ever been involved in.

"They have been bitter and personal before, but I think by far this one has been the most personal in terms of how it has been directed," she said.

"The last five years have been incredibly difficult and incredibly personalised. But you know what, I am a big girl and I have been in politics for a long time and if people think that bullying me is going to have any impact on how I go about doing the job then they seriously misunderstand who they are dealing with."

Mr Robinson insists personal attacks are not part of his style of politics. But he claimed there was anger among voters at the stance taken by Alliance on a number of issues.

"I haven't found any bitterness in the political discourse, and I think that's good," he said.

"There's nothing about my politics that want to make any part of this campaign a personal campaign because I think what we are setting out is a much greater stall for Northern Ireland and what we can deliver for Northern Ireland through our plan.

"So it doesn't need to be (bitter), I don't want it to be. But I do suspect and I do detect that there's a lot of passion on the doors, that's where you might find the bitterness and that's where you might find the vitriol because people feel let down."

While East Belfast is overwhelmingly Protestant its social composition is very diverse.

There are the working class loyalist communities around the old shipyards, where deprivation and unemployment hit hard, but also the leafy suburbs of Cherryvalley, Stormont and Knock where some of Belfast's most well-to-do families reside.

The constituency is broadly split half and half in regard to what would be viewed as working class and middle class areas.

Prof Shirlow noted past voting trends showing the DUP out-polling Alliance two to one among working class voters, with the ratio reversed in middle class communities.

"One factor that helps Naomi Long is that middle class people when they say they are going to vote in a certain way they usually stick with that," he said.

"With working class communities people will say 'yes, I am going to vote x y and z' but they are actually less likely to get up on the day and vote."

Canvassing on the Belmont Road last week, one of the constituency's more affluent areas, Mr Robinson said the reaction he received flew in the face of the notion it is Alliance territory.

"It's been incredibly encouraging," he said.

Mrs Long, for her part, insists she also has received a positive response from all parts of the constituency.

Only May's poll will determine whose assessment of the electoral mood was more accurate.