As the Board of Elections prepares to rule today on the validity of ballot-access petitions in the mayoral race, prospects are grim for opponents of Mayor Bloomberg. According to the clerk’s reports sent to the candidates and petition objectors yesterday, Mr. Bloomberg’s Republican challenger, Thomas Ognibene, is likely to be knocked off the GOP ballot, and the mayor will apparently withstand challenges to his Independence Party signatures and remain on that party’s line.

To force a Republican primary, Mr. Ognibene, a former City Council minority leader from Queens, needed to obtain 7,500 petition signatures from enrolled Republican voters requesting that he be placed on the ballot. In June, before the start of the five-week petitioning season, Mr. Ognibene said he hoped to collect twice that number to build an ample cushion against challenges to his signatures by the mayor. Last month, however, Mr. Ognibene filed only 8,116 signatures with the Board of Elections, and last night it appeared that his concerns about objections would come to fruition.

Mr. Ognibene said late yesterday that, according to the Board of Elections report sent to him by the clerk examining challenges by the Bloomberg campaign to his petitions – a process conducted at Staten Island – the review had determined that 2,379 of those signatures were invalid, placing Mr. Ognibene 1,763 short of the 7,500-signature threshold.

The official ruling on Mr. Ognibene’s petitions will come today at a noon hearing of the Board of Elections, but the parties involved said they expected the board’s decision to rubber-stamp the clerk’s findings. A spokesman for the board, Christopher Riley, said: “Nine times out of 10 … the commissioners are going to adopt the clerk’s report.” Mr. Riley declined to comment on the specific ballot challenges.

Mr. Ognibene, who is a lawyer, said that while he thought he could restore around 600 of the invalidated signatures, he would still fall short of the 7,500 mark. Appearing on the Republican ballot in September, he said, would now depend on persuading a federal judge that New York’s ballot-access rules pose an unfair disadvantage for Republicans, who, while outnumbered by Democrats in the city by a ratio of more than 5-to-1, must still collect the same 7,500 signatures as their Democratic rivals to appear on the ballot.

“We knew from the beginning that our salvation was in two areas,” Mr. Ognibene said. “One, federal court, and two, the Conservative Party line.” The former councilman has pledged to run as a Conservative Party candidate through the general election in November, promising to remain a gadfly to the mayor regardless of the outcome of his bid for the Republican nomination.

A veteran city political consultant, Jerry Skurnik, said yesterday of Mr. Ognibene’s chances: “I would think it’s a long shot, but it’s not impossible that he could get back on.” Federal judges, Mr. Skurnik said, have significant latitude in their decisions, and Mr. Ognibene’s fate would ultimately be “luck of the draw” in terms of which judge hears his case.

“There are two judges in the Eastern District known to be very liberal in their interpretation of election law,” Mr. Skurnik said.

Meanwhile, as it appeared yesterday that Mr. Bloomberg had secured the Republican nomination outright, efforts to strip him of the Independence Party line seemed unlikely to succeed. According to the clerk’s report received by the former Independence Party official challenging the mayor’s petitions on that line, Michael Zumbluskas, 439 of the 4,839 signatures were deemed invalid. Mr. Bloomberg needs around 3,900 signatures to appear on the Independence Party ballot.

Mr. Zumbluskas, however, said he hopes he will succeed in invalidating another 513 signatures as a result of fraud. The clerk’s report said it did not rule on 291 purportedly forged voter signatures and on another 222 petitions with allegedly forged witness signatures.

While the forgeries are technically “referred to the board” on the clerk’s report, Mr. Skurnik said that determining forgeries is an evidentiary process – one that requires eyewitnesses or the testimony of a handwriting expert – and would be handled only by a court. Even then, the consultant said, “It’s very rare that people are thrown out on forgeries because it’s hard to prove.”

The chairwoman of the New York County Independence Party, Cathy Stewart, said she was confident Mr. Bloomberg would appear on the Independence line.

“There’s not forgery and fraud,” Ms. Stewart said. “It’s a good petition.”

Mr. Zumbluskas said yesterday he has a court date tomorrow morning in Manhattan for a ruling on his challenges, although it remained unclear late yesterday evening whether Mr. Zumbluskas had filed the requisite paperwork to obtain a full review of all 4,839 Independence Party signatures for the mayor.