Okay, granted, there's apparently less to this than meets the eye, or at least the headline ("nearly toppled Romney"). The story (nicely reported by Joshua Green) is apparently that in the run-up to the Michigan primary, the Santorum campaign pushed the disgraced former Speaker to drop out and endorse the struggling defeated Pennsylvania Senator. Newt, with typical Newtness, instead tried to get Santorum to go for a "Unity" ticket with Newt on top. The negotiations got as far as face-to-face meetings between the candidates, but eventually fell apart.
Now, this was after Santorum's big day on February 7 in which he posted surprise victories in Colorado and Minnesota caucuses and a Missouri beauty contest...it was at the high point of the Santorum campaign. But it was also a month after Romney had pretty much locked up the nomination. More to the point, there was no large Gingrich constituency ready to follow him to Santorum; to the contrary, if they had really named a Santorum-Gingrich ticket it's extremely likely that Santorum would have drawn fire from the large portion of the GOP who didn't want Newt anywhere close to the White House.
The truth is that if party actors wanted Rick Santorum, they would have rallied to him, and they very much did not. They did not after his win in Iowa; they did not after his CO/MN/MO big day. Even in the extremely unlikely even that a Newt endorsement would have been enough to push him past Romney in Michigan (Santorum lost 41/38 there, with Newt pulling 7%), Romney still would have won the day by winning the Arizona primary. And party actors still wouldn't have rallied to Santorum. Especially not to Santorum/Gingrich.
As for Gingrich/Santorum...well, if there's one thing that was clear from the beginning of the nomination process to the end it's that practically everyone who ever knew or worked with Newt Gingrich thought he would be a simply awful presidential candidate, much less president. And while many of them were willing to play along up to a point, every time he had any momentum at all they came out of the woodwork to make sure that whatever debacle Republicans would suffer through in 2012, at least it wouldn't be that debacle.
I'll give the last word to Jonathan Chait:
This is not surprising: Gingrich is always making elaborate historical arguments, and they always seem to justify his political self-interest at any given moment. If Santorum were really clever, he would have accepted the vice-presidential spot and waited for the inevitable Gingrich impeachment — misappropriating funds for jewelry? starting a war with Mars without Senate approval? declaring himself president for life? all the above? — and taken over then as a comparatively reassuring figure. Sadly, the world will never know.