FACTBOX: What next in Zimbabwe's run-off election?

(Reuters) - Zimbabwe's opposition Movement for Democratic Change said on Friday it was considering pulling out of the June 27 presidential run-off election.

Party leader Morgan Tsvangirai is due to decide on Monday whether he will contest against President Robert Mugabe.

The second round follows a disputed March 29 election in which official results showed Tsvangirai beat Mugabe but not by enough votes to avoid a run-off.

Below are answers to some questions on the vote:

WHAT HAPPENS IF THE OPPOSITION WITHDRAWS?

Mugabe would be sworn in for a new five-year term in office, but he would face problems governing and passing legislation. ZANU-PF lost control of parliament to the opposition for the first time in a parallel vote on March 29.

Zimbabwe's economic crisis is likely to worsen, and analysts say there will be few prospects of serious political change. The exodus of millions of migrants to neighboring countries, including South Africa, could accelerate.

The risks that poses for the region were highlighted by recent attacks on African migrants in South Africa.

The Zimbabwean leader and his senior officials could be slapped with a new round of targeted sanctions by the European Union, Britain and the United States, which have criticized his authoritarian rule and handling of the election campaign.

WHAT IS HAPPENING IN THE CAMPAIGN?

Since Tsvangirai returned to Zimbabwe on May 24, he has been detained five times by police while on the campaign trial. His lieutenant, Tendai Biti, is in custody facing treason and other charges.

The MDC says at least 70 of its supporters have been killed by ZANU-PF militia and security forces and thousands of anti-Mugabe opponents and voters have been beaten or harassed. Mugabe blames the opposition for the bloodshed.

The Zimbabwean leader has branded the MDC a puppet of Britain and the United States and vowed to never allow it take power. State media has virtually ignored the MDC campaign and refuses to run its advertisements.

HOW FAIR WILL THE VOTE BE?

The MDC, rights groups, some African nations, the United States and former colonial power Britain have said they do not believe the poll would be free and fair because of the escalating violence.

The Southern African Development Community, a regional grouping of 14 nations including Zimbabwe, has sent about 400 observers to monitor the run-off. No observers from Western nations are being allowed into Zimbabwe for the poll.

British Prime Minister Gordon Brown has urged Zimbabwe's government to admit international rights observers and the U.N. envoy for human rights for the poll.

WHO IS LIKELY TO WIN A RUN-OFF?

On the face of it, Tsvangirai would appear to be in a dominant position to win a free election after defeating Mugabe in the first round.

But the climate of violence and intimidation makes this prospect difficult. Many pundits expect Mugabe to be declared the winner.

WHAT IF TSVANGIRAI WINS?

Pledges from the United States and other Western nations to inject billions of dollars into Zimbabwe if there is a new, government that embraces democracy and free markets could tempt voters to back Tsvangirai.

The MDC leader has not spelled out how he would ease Zimbabwe's economic crisis -- marked by severe shortages of basic goods and the world's highest inflation rate of 165,000 percent.

WHAT IF THERE IS PROLONGED POLITICAL DEADLOCK?

The MDC and trade unions could call for street protests to pressure Mugabe's government to step down, raising the prospect of a worsening of the political violence that has gripped the nation in the past three months.

Zimbabwe could slip at least briefly into the kind of violence that affected Kenya after its disputed December election, although Mugabe's security forces are likely to quickly end unrest.

Talk has been floated of a possible national unity government, but each side would insist on leading such an administration.

(Reporting by Harare and Johannesburg bureaux)