While I believe it’s possible for born-again Christians and pro-life Catholics to work together somewhat, evangelicals have allowed Catholics to represent them far too often. There are the “conservative Catholics” on the Supreme Court, as well as Rick Santorum, Jeb Bush, Marco Rubio, etc. It’s not like these politicians have an ability to attact Catholics in general. The only ones who vote for them are Catholics who would vote Republican anyway. So why is Bible-believing Christianity represented by almost noone but Catholics today? It seems to be because even conservative Catholics still don’t hold to the Bible as their highest authority.
On matters of moral issue I think your right. On matters of spiritual issues there are issues that cannot be overcome.
So if we want to march together at a pro-life rally, there’s nothing wrong with that. If we want to get together for an ecumenical prayer-fest, then one really needs to examine what they’re doing.