By Matt Siegel
SYDNEY, Feb 20 (Reuters) - Voters in Western Australia are headed for a rerun of last year's federal Senate race, after the High Court on Thursday voided the election over 1,370 ballots that disappeared during a recount of the tight contest.
The ruling could have an impact on conservative Prime Minister Tony Abbott's agenda. His Liberal-National coalition won a strong majority in the lower house of parliament in last year's poll, but lacks a majority in the upper house, or Senate.
Justice Kenneth Hayne invalidated the race because the lost ballots far exceeded the margins of error in two of the senate races, one of which came down to as few as 12 votes.
Senator Michael Ronaldson, the Liberal Party minister responsible for the Australian Electoral Commission, warned the agency over a replay of the debacle in the new vote, which could take place within weeks.
"It is incumbent upon the Australian Electoral Commission to ensure that never again will such a failure of process occur," he said in a statement.
Abbott campaigned on a promise to repeal an unpopular carbon trading scheme initiated by former Prime Minister Julia Gillard's Labor government.
He lacks, however, the votes to push its repeal through in the new Senate, in which the balance of power will be held by a handful of independent and single-issue senators.
Although his Liberal Party won three of the state's six Senate seats in the September vote, at least one of those seats is vulnerable in a fresh contest, says Antony Green, an election analyst for the Australian Broadcasting Corporation.
"If the Liberal Party were to lose a seat at the re-election, it would weaken the government's position in the Senate and strengthen the hand of the alliance of minor parties that has developed around the Palmer United Party," he told the Sydney Morning Herald Newspaper.
Due to a quirk in Australia's parliamentary system, the newly elected senators do not sit until July, four months after their colleagues in the lower house take up their positions.
(Editing by Robert Birsel)