Zimbabwe ? Evidence mounted Monday that Zimbabwe’s opposition candidate defeated President Robert Mugabe this past weekend in the first round of a national vote, creating the biggest threat to his grip on power in 28 years of unbroken rule.

Though official results remained mysteriously unannounced, the Zimbabwe Election Support Network, an independent observation group, said that a statistical model drawing on a sample of posted vote tallies showed that Mugabe got 41.8 percent of the vote, compared with 49.4 percent for opposition leader Morgan Tsvangirai. An independent, Simba Makoni, got 8 percent, the group found.

If confirmed, the monitor group’s numbers would push Mugabe and Tsvangirai into a runoff vote – something analysts have long said would consolidate opposition to the president and hasten the end of his rule. Zimbabwe election laws require that a winning candidate get more than 50 percent of the vote. The new election likely would be April 19.