Republican mayoral candidate George McDonald wants Manhattan GOP leader Dan Isaacs to resign after appearing on tape weighing (but not accepting) a thinly disguised bribe from an undercover FBI agent posing as an associate of state Sen. Malcolm Smith.

The discussion centered on a quid pro quo to deliver Smith the GOP ballot line for mayor.

Isaacs never accepted any bribe and has not been charged in the far-reaching Smith bribery case, and is believed to be cooperating with the feds. The ongoing Manhattan probe by US Attorney Preet Bharara has led to the indictments of Smith, ex-Bronx GOP chairman Jay Savino, Queens Councilman Dan Halloran and former Queens GOP vice chairman Vincent Tabone, among others.

But McDonald said Isaacs has to go after seeing last week’s Post report detailing taped exchanges between Isaacs and an FBI undercover posing as a real estate-developer pal of Smith during a Feb. 14 luncheon at Spark’s Steak House. During the discussion, Isaacs, who is backing John Catsimatidis for mayor, talks up Smith and says, “I’ll send you a retainer agreement” when the undercover offers him work in exchange for getting Smith on the GOP ballot.

“We need party leaders that are not for sale to the highest bidder, Issacs needs to step down and Catsimatidis needs to ask for his donations back [from the Manhattan GOP],” McDonald said. The prosecution said such disclosures should not be used to smear the character of someone not charged in the case, or hinder an ongoing criminal investigation.

“The Republican Party needs to clean up its act or risk becoming no better than the party of Weiner, Spitzer and Smith,” he added.

Isaacs has repeatedly maintained he did nothing wrong.

Meanwhile, Bharara’s office filed a motion seeking a gag order from the court to prevent defendants or their lawyers from leaking sensitive information in the case with third parties or the media.