In 1964, Tan contested both the Parliamentary seat of Batu in Kuala Lumpur and the Selangor State Assembly seat of Kepong. Although the Alliance crushed the opposition, Tan won both seats – with a margin of victory of two votes for his Parliamentary seat, officially the smallest ever margin of victory in a Malaysian election as of 2004 – and was one of only two Labour members of parliament (MPs). Tan would not relinquish either seat until his retirement. During his tenure as an MP, Tan became known for thoroughly probing government policies and making his views known. As a result, he was popularly known as "Mr. Opposition". Tan was known for his opposition to wanton amendments of the Constitution in Parliament, and also for his outspoken opposition to the establishment of Malaya (and later Malaysia) as an Islamic state, which he felt contravened the Constitution. He was also known for his devotion to his constituents, making himself available to them at least two days a week in his office. Tan made no secret of his religion, and freely peppered his Parliamentary speeches with quotations from the Bible, leading the first Malaysian Prime Minister, Tunku Abdul Rahman, to remark how much he sounded like a preacher.