If the ultranationalists become the main opposition in Hungary, it is unlikely Sandor Kepiro will ever be brought to justice

This Sunday, Hungarians will vote in a national election that according to all the polls will result in a change of power. After eight years of rule by the Hungarian Socialist party, it appears that the conservative opposition party Fidesz (Alliance of Free Democrats) will be taking over, apparently by a wide margin. With their victory a foregone conclusion, the most important question facing the Hungarian electorate is the fate of the relatively new extreme rightwing party Jobbik, which until now has not been represented in the local parliament, but won several seats in the last elections for the European parliament.

A strong showing by these ultranationalists, along with the possible virtual disappearance of the Socialists, whose administration is generally considered to have been a failure in most respects, would make Jobbik the main opposition party with potentially very dangerous consequences, especially for the country's minorities. Jews and, in particular, the Roma have been the primary target of the party's attacks since its establishment, both of whom were victimised by the Nazis and their Hungarian collaborators – nostalgia for whom is a strong component of Jobbik's message.

One of the persons who should be watching the elections with great interest is an elderly former Hungarian gendarmerie officer, whose legal fate might well be decided by Sunday's results. I am referring to Dr Sandor Kepiro, who on the very same day will be named by the Simon Wiesenthal Centre as number one on our new annual list of the world's "most wanted" Nazi war criminals, which is published on Yom Hashoa, Israel's Holocaust memorial day.

Kepiro was one of the officers who organised the mass murder of at least 1,200 civilians (mostly Jews, but also Serbs and Gypsies) in the city of Novi Sad, then under Hungarian occupation, on 23 January 1942. On that day, the men under his command rounded up hundreds of civilians in the centre of town, who were taken to Sokolski Dom (a local theatre), where a committee of Hungarian officers decided who would live and who would be sent to the nearby banks of the Danube River, where they would be executed by teams of Hungarian shooters.

The Kepiro case is very important for several reasons. First of all, the facts regarding the massacre are not in dispute, and Kepiro admits that he was in Novi Sad on that day in his capacity as an officer. Second, he and his fellow officers were actually convicted in Hungary in January 1944 in conjunction with the events in Novi Sad (although not for murder but rather for violating their code of honour), but had their convictions quashed and were given promotions following the Nazi occupation of the country in March 1944.

One of the facts that emerged during that trial was that when Kepiro, who was already a lawyer at the time, received his orders prior to the roundups he asked for them in writing, as he obviously realised they were immoral and hence illegal. The response was that such orders were only transmitted verbally and Kepiro complied regardless, making him, in my opinion, a "moral monster of the Holocaust": a highly educated person who fully realised that what he was asked to do was against the law but did so anyway. The fact he was an officer with command responsibility and that his actions played a role in a mass murder ,which took the lives of so many people (new research claims that the number of victims was as high as 3,000, but this figure has not yet been confirmed), are the factors that make Kepiro our number-one suspect.

I found Kepiro living in Budapest in the summer of 2006. (He had fled Hungary in 1945 and had lived in Argentina for 48 years, prior to returning to the Hungarian capital unnoticed and undisturbed by the authorities.) At that time, I was fairly confident he would be prosecuted shortly after for his crimes. Contrary to my expectations, however, that has not yet happened. Initially, I asked that his original sentence be implemented, but six months later I was informed that since a Hungarian court had officially cancelled his conviction, that could not be done. Instead, a new investigation against him was initiated in February 2007 and has yielded considerable incriminating evidence.

So why is he still a free man? One possible explanation is that the authorities hoped he would die and spare them the trouble and embarrassment of a trial which would highlight Hungarian complicity in Holocaust crimes; but Kepiro was uncooperative in that respect and even at the advanced age of 95, he is in relatively good health and has on numerous occasions given media interviews.

Since he is still very much alive, it boils down to politics, which is why Kepiro will no doubt be watching the election results very carefully. Our experience has shown that very often leftwing governments, which are generally much more anti-fascist than their conservative opponents, are afraid to prosecute Holocaust perpetrators for political reasons, whereas the latter, whose nationalist credentials are much stronger, are not afraid to do so and in fact have much to gain in this respect, since the prosecution of a local Nazi war criminal strengthens their credibility on human rights.

Under these circumstances, Kepiro's fate will most likely be linked to that of Jobbik. If the ultranationalists become the main opposition party, it will be hard for a Fidesz government to put Kepiro on trial. If they do not have to worry about Jobbik's reaction, it is likely that Kepiro will finally be held accountable for his role in the massacre in Novi Sad. A case like his should not, of course, be dependant on politics, and the Hungarians will no doubt proclaim the independence of their judiciary – but the facts speak for themselves, not only in Hungary but all over the world. At this point in time, it is political will that will probably be the key factor in determining how many, if any, of the criminals on our most wanted list will ever be brought to trial.

guardian.co.uk