Senator John McCain of Arizona is poised to receive the backing of Mitt Romney, who was his most formidable main rival and most heated opponent this year, later Thursday afternoon, officials in both campaigns said.

With his Valentine’s Day endorsement, Mr. Romney will release his delegates – and while the rules vary by state, he can ask many of them to support Senator McCain...

The formal announcement is expected to come at 4 p.m. at Mr. Romney’s campaign headquarters in Boston.