Nearly half of Cambodia's opposition parliamentarians have fled abroad in the past month to escape a government crackdown that has battered their party with legal cases, an MP said on Wednesday.The exodus further imperils the prospects of a party that poses the only viable challenge to premier Hun Sen in next year's national poll.The strongman has smothered dissent in the lead-up to the election and is accused of picking off his critics through a series of dubious court cases.