ALBANY, N.Y. (AP) _ Backers of political extremist Lyndon LaRouche were barred today from seeking the Democratic nominations for governor, lieutenent governor and U.S. Senator from New York.

Steven LaFever, spokesman for the state Board of Elections, said the three LaRouche candidates would appeal the ruling by Election Commission Donald Rettaliata. A court hearing on the appeal was set for Friday, he said.

Rettaliata ruled that the joint nominating petitions for the three candidates, containing 58,790 signatures, were invalid because the 25 petitions failed to list on their cover sheets the number of signatures collected for each candidate.

The candidates are Judah Philip Rubinstein for governor, Fernando Oliver for lieutenant governor and Webster Tarpley for U.S. Senate. The primary is Sept. 9.

Gov. Mario Cuomo, endorsed by the state Democratic Convention to run for a second term, has been one of the leaders in a campaign by Democrats to bar the LaRouche supporters from the party primary.

Cuomo has denounced the LaRouche candidates as anti-Semitic and hostile to the traditional goals of the Democratic Party, charges they deny.

LaRouche supporters surprised Democrats around the nation earlier this year when they won primaries for lieutenant governor and secretary of state in Illinois.

While Cuomo supporters raised a host of technical objections to the petitions by the LaRouche candidates, only the cover sheet issue was found to warrant barring the candidates from the ballot, LaFever said.

The three candidates have been waging low-budget campaigns around the state, spreading LaRouche's views on alleged worldwide drug conspiracies by public officials, quarantines for AIDS victims and the establishment of settlements elsewhere in the solar system.