The “major endorsement” that Sen. Eric Schneiderman’s campaign is touting that will come at noon today in front of the Columbus Circle entrance to Central Park is the man the Manhattan senator is seeking to replace: AG Andrew Cuomo, according to a source with knowledge of the event.

There has been considerable speculation about when – and even if – the gubernatorial candidate would give Schneiderman his blessing, particularly after he did not endorse prior to the primary and made it clear during that five-way fight that he would prefer someone other than Schneiderman to win.

It will be interesting to hear how Cuomo explains that Schneiderman is not, as the AG’s many surrogates intimated (and several of Schneiderman’s primary opponents charged outright), in fact too much a creature of Albany to be the state’s top attorney at at a time when Cuomo is running on a platform of cleaning up Albany.

Schneiderman, who told me during a CapTon interview last week that he and Cuomo were stringing out this “will he/won’t he” act to add some “drama” to the race, did a good job during the primary of boxing the AG, recognizing that if he couldn’t land Cuomo’s endorsement, the next best thing would be preventing him from giving it to someone else – especially his closest competitor, Nassau County DA Kathleen Rice.

As it turned out, Schneiderman beat Rice on Sept. 14 by only 2 percentage points, and we’ll never know if Cuomo’s seal of approval would have been sufficient to push her into the lead.

Schneiderman lined up a long string of endorsements from the left – from labor unions like SEIU 1199 and HTC to the Rev. Al Sharpton (a nod that might come back to haunt the senator in the general election against Republican Staten Island DA Dan Donovan, after he pledged Sharpton’s National Action Network would have an “annex” in Albany if he’s elected”).

Cuomo might be playing to the middle, but he also doesn’t want to alienate the liberal base that is aligned behind Schneiderman. Schneiderman’s campaign also pressed Rice hard on a number of her negatives – most notably her failure to vote for close to two decades – and her camp was flat-footed in responding.

Cuomo has repeatedly said he would support the Democratic nominee in the AG race, but took his sweet time in doing it. (Eliot Spitzer, who didn’t want Cuomo to win the 2006 primary campaign, endorsed his erstwhile political rival immediately after he won his four-way battle four years ago and quickly cut a commercial for him, too).

This is going to put Cuomo at odds with one of his major endorsers, Mayor Bloomberg, who is solidly behind Donovan.

Bloomberg allies tried to pitch Donovan to Cuomo, saying he would be a better partner in the AG’s office for Cuomo (if and when he becomes governor) because he’s not an Albany insider and would therefore be more able to help clean up legislative corruption.

But Cuomo didn’t bite. Again, crossing party lines to back an opponent to someone the base has rallied around would have likely have proved politically untenable to Cuomo.

Today’s announcement leaves state Comptroller Tom DiNapoli as the odd man out. Cuomo has steadfastly refused to endorse the former Long Island assemblyman, saying his pay-to-play pension fund investigation prevents him from doing so.

DiNapoli has a lot of traditional Democratic base support too, but his most significant backer is Assembly Speaker Sheldon Silver.

It remains to be seen if DiNapoli’s camp manages to pressure Cuomo into getting off the fence about the comptroller – especially since his GOP opponent, Harry Wilson, has refused to endorse his party’s gubernatorial nominee, Carl Paladino, and has been saying nice things about Cuomo, even once calling the AG his “running mate” on the Independence Party line.