EXCLUSIVE

TMZ has learned Mitt Romney's son Tagg -- who had twins this year through a surrogate -- signed an agreement that gave the surrogate, as well as Tagg and his wife, the right to abort the fetuses in non-life threatening situations ... and Mitt Romney covered some of the expenses connected with the arrangement ... and it may boil down to an incredibly stupid mistake.

The twin boys -- David Mitt and William Ryder -- were born on May 4, 2012. We've learned Tagg and his wife Jen, along with the surrogate and her husband, signed a Gestational Carrier Agreement dated July 28, 2011. Paragraph 13 of the agreement reads as follows:

"If in the opinion of the treating physician or her independent obstetrician there is potential physical harm to the surrogate, the decision to abort or not abort is to be made by the surrogate."

Translation: Tagg and Jen gave the surrogate the right to abort the fetuses even if her life wasn't in danger. All the surrogate has to show is "potential physical harm," which could be something like preeclampsia -- a type of high blood pressure that could damage the mother's liver, kidney or brain, but is not necessarily life-threatening.

Paragraph 13 goes on:

"In the event the child is determined to be physiologically, genetically or chromosomally abnormal, the decision to abort or not to abort is to be made by the intended parents. In such a case the surrogate agrees to abort, or not to abort, in accordance with the intended parents' decision."

And there's another relevant provision in Paragraph 13:

"Any decision to abort because of potential harm to the child, or to reduce the number of fetuses, is to be made by the intended parents."

Translation: Tagg and his wife, Jen, had the right to abort the fetuses if they felt they would not be healthy.

Sources connected with Mitt Romney tell TMZ, Mitt was involved in the surrogate arrangement because he paid some of the expenses connected with the agreement. We do not know if Mitt Romney read the contract or knew the terms.

Mitt has said, "I'm in favor of abortion being legal in the case of rape and incest and the health and life of the mother." Otherwise, Romney is against abortion.

Now for the stupid mistake. We've learned Tagg chose the same surrogate in 2009, who gave birth to a boy. Attorney Bill Handel -- a nationally-known expert in surrogacy law who put the deal together between Tagg and the surrogate -- tells TMZ when the 2009 contract was drafted there was no Paragraph 13 providing for abortion because Tagg and his wife didn't want it.