By MacDonald Dzirutwe

HARARE, March 27 (Reuters) - Zimbabwe's Morgan Tsvangirai goes into Saturday's presidential election knowing another defeat could end a political career that has brought him closer than anyone to unseating President Robert Mugabe.

Once hailed as the great hope of Mugabe's foes, the fiery trade unionist goes into the election with opposition ranks divided and with a defector from the ruling party sowing further confusion by running against Mugabe as an independent. The gruff Tsvangirai emerged eight years ago as the first serious threat to the veteran leader, now 84, but a split in his Movement for Democratic Change in 2005 seriously dented his image and standing.

"For Tsvangirai this is not just an ordinary presidential election, he will be seriously thinking about his political future if he were to lose," said John Makumbe, a University of Zimbabwe political science lecturer and Mugabe critic.

Economic analysts remain sceptical of Tsvangirai's ability to revive an economy that was once an African success story, saying he has neither the experience nor the policies to do so.

Mugabe frequently labels Tsvangirai a "pathetic puppet" used by one-time colonial power Britain to try to bring him down.

The former trade union leader says he is his own man with popular support and calls Mugabe a violent tyrant.

Tsvangirai was hospitalised a year ago and said he had been bashed in police custody, an event which his critics say helped revive his sagging political fortunes.

He has vowed to defeat Mugabe this time around, saying the veteran leader cheated him of victory in 2002.

Tsvangirai's working-class roots contrast with Mugabe's background as a former guerrilla leader who has a string of university degrees.

Tsvangirai, 56, is the self-taught son of a bricklayer. He worked in a rural mine to support his family and cut his political teeth in the labour movement as a mine foreman.

In 1988, he became full-time secretary general of the Zimbabwe Congress of Trade Unions. Under his leadership, the federation broke ranks with Mugabe's ZANU-PF.

Tsvangirai led paralysing strikes against tax increases in December 1997 and twice forced Mugabe to withdraw announced hikes. He helped found the labour-backed MDC in 1999.

In February 2000, the MDC engineered Mugabe's first poll defeat -- the rejection in a national referendum of a new draft constitution that would have entrenched his presidential powers.

That June, despite killings and police intimidation, the MDC stunned ZANU-PF by winning 57 of the 120 seats at stake in a parliamentary election as Tsvangirai captivated the public with powerful speeches. (Editing by Matthew Tostevin)