Behind the flag: Mary Lou McDonald is standing in Dublin Central

I was in the RDS late on the night that the count for Dublin Central was concluded in the 2007 general election. A forlorn Mary Lou McDonald stood alone in the entrance of the count centre, no doubt contemplating how her own, and Sinn Féin's, rosy prospects a few weeks earlier had been so roundly dashed. It is generally accepted that the demolition by then PD leader Michael McDowell of Sinn Féin's economic policy, as espoused by Gerry Adams during the secondary leaders' debate, was the catalyst that destroyed those prospects.

Yet almost four years later McDowell and his party are no longer in politics while Gerry Adams has the genuine prospect of occupying the seat in Dáil éireann currently held by Enda Kenny, flanked by a cohort of Sinn Féin deputies including the aforementioned McDonald.

The Red C poll in January 2010 showed Fianna Fáil to be at 27% while Sinn Fein was at 8%. On 7 January 2011 both parties were tied at 14%. Sinn Féin is a party on the rise while a battered and bruised Fianna Fáil is trying desperately to arrest a slide – if not to political oblivion, then certainly to exclusion from power for a considerable period of time. Sinn Féin has been buoyed by its spectacular success in the Donegal South-West by-election and is now in the process of manoeuvring itself into the position of champion of the significant section of the electorate that feels betrayed and deeply damaged by the budget measures taken by the outgoing government.

The Sinn Féin message is quite simple. It would reverse the universal social charge, restore the minimum wage, severely increase tax on the wealthy and make the banks pay for the economic debacle rather than the ordinary taxpayer. Its strategy may be populist and indeed many of its policies are unachievable, but nonetheless, it is finding a high degree of resonance, particularly among those in the urban and commuter belts.

Many of those who supported Fianna Fáil in 2007 because they could buy a house, a better car and had a good job now have negative equity on a home that they are struggling to meet mortgage repayments on, and they may well have lost their job. To those people, Sinn Féin has a message that gives them hope. Throughout history, in the midst of economic crises such as this one, the political middle ground contracts as people turn to the extremes for solutions.

It is not just Fianna Fáil that Sinn Féin is targeting. In the past few weeks it has singled out Labour, which had regarded itself as the leader of the left. For the past two years, Labour also articulated what many observers regarded as populist policies, but because this was over such an extended period, its impact had begun to wane, particularly as Labour's expectation was for an election months ago. Sinn Féin cleverly ensured the holding of the by-election and has roundly opposed the need to pass the finance bill. It has timed its run to perfection and would appear to be outflanking Labour for the vote of the disenchanted, particularly in urban areas.

While Labour appears to have belatedly recognised this, Sinn Féin has undoubtedly spooked the party, as seen by the tetchiness of its spokespersons in the past week when confronted by Sinn Féin representatives. Labour has good reason to be worried, because in June 2010 it led Sinn Féin by 19% points in a Red C poll. Three weeks ago that gap had been reduced to 7% points.

In order to be the main opposition party, Sinn Féin needs to get more seats than Fianna Fáil. Both parties are tied at 14% but compared to 2007 Fianna Fáil is down 66.7% while Sinn Féin has increased by 101.7%. Applying this change to each constituency, Sinn Féin is better placed because of the FF candidate strategy. As of last Tuesday, Fianna Fáil had selected candidates in 29 constituencies and in 20 of those it selected more than one candidate. Even in constituencies where Fianna Fáil could push up to 20%, the effect of two or more candidates will dilute the value of the split first-preference vote and it will trail behind Sinn Féin. Sinn Féin has candidates selected in 35 constituencies but in only three did it select more than one candidate and in one of those, Cavan/Monaghan, it is targeting and will almost certainly get two seats. Sinn Féin will also have the advantage of attracting more second- and subsequent-preference votes than Fianna Fáil.

If Sinn Féin continues to run, at worst, neck and neck with Fianna Fáil, it will almost certainly win seats in Cavan/Monaghan (two), Cork North-Central and both Donegal constituencies. In Dublin, it will win in Central, Mid-West, North-East, North-West, South-Central and South-West. It will also win seats in Kerry North/Limerick West, Louth, Meath West, Roscommon/South Leitrim, Sligo/North Leitrim and Wexford. It has realistic chances, particularly if it continues to eat into the Labour vote, in Cork East, Cork South-Central, Laois/Offaly, Mayo, Waterford and Wicklow.

That means Sinn Féin is in genuine contention for a minimum of 23 seats, and once a party is on a roll, as was Labour in 1992, other seats not listed above could be won. With these sorts of numbers it is difficult to see what could prevent Sinn Féin from becoming the main opposition party.

What is certain is that from now to election day no politician in their right mind should underestimate Sinn Féin.