ISLAMABAD, August 1 (Reuters) - Pakistani lawmakers on Tuesday elected former petroleum minister Shahid Khaqan Abbasi as the country's new prime minister, replacing ousted veteran leader Nawaz Sharif. Sharif's ruling PML-N party used its majority in the National Assembly to install Abbasi, after the Supreme Court last week ordered Nawaz Sharif disqualified from office over failure to disclose a source of income. Abassi's tenure is expected to be short, as Sharif has earmarked his brother Shahbaz as the successor once he becomes eligible. Shahbaz, currently the chief minister of the vast Punjab priovince, needs to win a parliamentary by-election before other lawmakers can elect him as prime minister.