By Ralph Boulton ISTANBUL (Reuters) – With a huge rally on the approach to the Bosphorus waterway, Turkish Prime Minister Tayyip Erdogan will launch a final electoral push on Sunday to stifle rivals he described this weekend as an alliance of evil in cahoots with terrorism. Erdogan, who vowed last week to wipe out Twitter in Turkey for carrying anonymous postings accusing him and associates of corrupt dealings, accused the networking site of systematic character assassination. The site remained blocked on Saturday in Turkey, access denied as a protection measure. Leading condemnation from Western governments and rights groups, the White House said the Twitter ban ahead of March 30 local elections undermined democracy and free speech.