LISBON — To stop the explosive growth of the ultranationalist Chega party, Portugal’s leading conservatives are doing the previously unthinkable: endorsing the center-left candidate for president. Last week, Portugal’s prominent center-right politicians are publicly backing António José Seguro — a former secretary general of the Socialist Party — in the runoff presidential election on Feb. 8. The conservative endorsement is a collective rejection of the opposing candidate, far-right Chega