The service was streamed to thousands of people using the Facebook Live video service and has been viewed more than 230,000 times.
Fr Frank Pavone, who performed the action, is part of the anti-abortion group Priests for Life. He said the foetus he used was entrusted to him by a pathologist for burial.
The 44- minute video also shows the church leader holding up graphic images of abortions and dead infants.
On his Facebook page he wrote: "Majority of voters in online poll choose pro-life as most important issue."
In the video he says: "I am showing [this dead child] to you because in this election we have to decide whether we will allow this child killing to continue or not.
"Hillary Clinton and the Democratic platform says yes... Donald Trump and the Republican platform says no."
There are calls from Catholic leaders for him to stand down.
In a blog post for Patheos, Scott Eric Alt argued Pavone's actions amounted to sacrilege.
He wrote: "I am going to be, excuse the pun, frank: What Fr Pavone did was a sacrilege.
"It is a violation of canon law, which states that the altar is consecrated for one purpose and one purpose only.
"It is consecrated for the Holy Sacrifice of the Mass.
"It is not consecrated so that a dead child can be placed there as part of a political stunt to lobby for a favoured presidential candidate."
Polling is open in the USA and Americans are selecting either Democratic nominee Hillary Clinto, or Republican nominee Donald Trump for president.
