President Robert Mugabe's grip on power has faced little recent threat from Zimbabwe's splintered opposition, but efforts to forge a unified alliance in next year's election could test his supremacy.

Any opposition bloc would first have to overcome fierce rivalries within their various camps, and then hold together in an intense and often violent political landscape dominated by Mugabe's ZANU-PF party.

Two deals between opposition parties signed this week represent an embryonic attempt to forge an alternative to Mugabe's rule, which began with independence from Britain in 1980.

Now aged 93 and in weak health, Mugabe has struggled to maintain control over ZANU-PF. The party has been hit by infighting and public frustration over the country's economic woes.

The strife presents an opportunity to the opposition only if they can unify, according to Alois Masepe, a Harare-based political analyst.

"It is a move in the right direction," he said of steps toward the alliance.

"A fragmented opposition does not help anyone except ZANU-PF." 

Anything less than unity would mean near-inevitable victory for Mugabe.

The going is tough, after decades of repression, vote- rigging and ZANU-PF violence during campaigning.

But another stumbling block is apathy.

"The opposition leaders must find strategies to get the people to participate," Masepe said. "The majority… don't." 

On Wednesday, Morgan Tsvangirai, leader of the Movement for Democratic Change (MDC), held a joint press conference with Mugabe's former vice president Joice Mujuru, who has set up the National People's Party.

"This is just the beginning," Tsvangirai said as the two signed a pact to work together to try and topple Mugabe in next year's poll.

The following day, Tsvangirai signed another pact to re- unite with the MDC-N, which split away in 2005, and is led by the influential Welshman Ncube.

The MDC has been weakened by repeated splits, and was badly fractured by Tsvangirai's troubled term as Mugabe's prime minister in a coalition government from 2009 until 2013.

"We owe it to future generations, we owe it to ourselves to do everything that we can to make sure that come 2018 we won't fail," Ncube said.

In 2008, Tsvangirai pulled out of a run-off against Mugabe after a wave of deadly violence, vote-rigging and voter intimidation from ZANU-PF operatives.

This week's pacts, however, are fledgling.

"More still needs to be done to unravel the matrix of cheating by ZANU-PF," warned Takavafira Zhou, a political scientist at Masvingo State University.