BRUSSELS -- European voters appear set to give some of their leaders a bloody nose Thursday and Sunday, taking the opportunity of European Parliament elections to vent their anger over the state of things at home. In Britain, Prime Minister John Major's Conservative Party is expected to suffer heavy losses to the opposition Labour Party in a ballot to be held Thursday. If the losses are particularly bad, Conservative activists say a serious challenge to Major's leadership later in the year is almost certain. Spanish Prime Minister Felipe Gonzalez, meanwhile, is looking at a possible election win by the opposition center-right Popular Party for the first time at the national level in 12 years. A particularly poor showing Sunday would put Gonzalez under pressure to call an early general election. On the same day, in Germany, voters will give Chancellor Helmut Kohl his first nationwide test since 1990 and provide a pointer to the likely outcome of general elections in October. Two recent polls showed Kohl's Christian Democrats either level with or a few percentage points ahead of Rudolf Scharping's Social Democrats. Officially at stake in the European elections are 567 seats in the European Parliament, which meets in the French city of Strasbourg. The parliament has slowly been gathering power in the European Union over the past few years and can now influence some legislation as far as rejecting it. It also has the power to turn down as a body the nominees chosen by member states to sit on the European Commission, the EU's legislative engine. But the elections, which begin Thursday in the Netherlands, Ireland, Denmark and Britain, are typically used by voters to express their views on incumbent politicians at home. "We have effectively 12 national elections with a slightly European flavor," said one EU diplomat. While the stakes are probably highest for Major and Gonzalez, voters in other countries are also expected to treat the poll as a referendum. It will be, for example, Italian Prime Minister Silvio Berlusconi's first test since hammering the left-wing opposition in March. A recent poll indicated that his Forza Italia would see an increase in support, while Berlusconi's government allies, the neo-fascist National Alliance and the federalist Northern League, would hold steady. Likewise in Greece, Socialist Prime Minister Andreas Papandreou will get a first feel for his popularity following his general election win last year. In at least one country, however, a leader is asking his supporters to use the elections for a different kind of referendum. Outgoing Prime Minister Ruud Lubbers urged Dutch voters Wednesday to turn out in force as a show of support for his candidacy to become president of the European Commission.