From the outside, at least, Mr Mahathir does not look like a politician with much to splutter about. Eighteen years and four elections after coming to power, he has established himself as the longest serving democratic leader in Asia. Two years after the Asian financial collapse, and against the predictions of many experts, the economy is already recovering.
But at a news conference yesterday, Mr Mahathir was asked about Anwar Ibrahim, his former deputy, who was sacked, arrested, and charged with corruption and sodomy last year. Did the Prime Minister believe that the affair had damaged his personal popularity?
"Go and shove it down your throat," Mr Mahathir responded. "I don't answer such stupid questions. It's stupid because only you would think of such things. We'll see what the election results are."
Mr Mahathir had spent the day in his safe constituency in Kedah, a state in the north-east of Malaysia, basking in the adulation of his many supporters. Why, then, did he sound so shrilly defensive?
Despite appearances, there is a great deal at stake in today's election. Since independence from Britain in 1957, Mr Mahathir's National Front coalition has grown used not only to power, but to overwhelming political dominance. A tame media, a divided opposition, a draconian internal security law and a powerful grass-roots political organisation have brought him the kind of power rarely enjoyed by democratic rulers.
In the outgoing parliament, only 22 seats out of 193 were held by the opposition; at the regional level, the National Front controlled 12 states out of 13. But despite the certainty of Mr Mahathir's victory, all that is beginning to change.
The key lies with the man whose name so upset the Prime Minister - Anwar Ibrahim. Until last year, the two were close friends; few doubted that when Mr Mahathir, 73, died or stepped down, Mr Anwar would take over. Then something changed. According to the Prime Minister, it was his discovery of his protege's promiscuous bisexuality. According to Mr Anwar, it was the challenge which the popular deputy was preparing to make for the National Front's leadership.
Mr Anwar's arrest, and his beating up by the chief of the Malaysian police, shook the trust of many of those who revered the Prime Minister for the prosperity and pride which he had brought to Malaysia.
It also had the effect of uniting the previously divided opposition. For the first time, Mr Mahathir faces a united coalition of four parties - the Islamic party Pas, the Chinese-based Democratic Action Party, the small Malaysian people's party and the National Justice Party lead by Mr Anwar's wife, Wan Azizah Wan Ismail, on behalf of her imprisoned husband.
Realistically, the Alternative Front, as it calls itself, can expect only modest gains tomorrow. At present, Pas controls the state of Kelantan, and stands a chance of gaining neighbouring northern states. Few but the most ardent optimists forecast victory for the opposition, but many hope to gain at least one-third of the seats, thus depriving Mr Mahathir of the power to change the constitution. In most countries, these would be glancing blows. But to Mr Mahathir, they are crushing.