The expected backing of the Arizona senator would lend more establishment support to Romney's campaign and potentially add to the sense of momentum following his ultra-narrow, eight-vote victory in the Iowa caucuses Tuesday.
Most recently, he supported Romney's decision to skip a planned debate that would have been hosted by Donald Trump.
Four years ago McCain skipped the Iowa caucuses and focused his efforts on New Hampshire, where he edged Romney by more than 5 percentage points in a victory that put him on the path to the nomination.
It was not clear which of Romney's planned events in New Hampshire on Wednesday that McCain would attend. Romney is scheduled to speak first at Manchester Central High School -- where President Obama spoke just more than a month ago. Later, he holds a town hall meeting in Peterborough, a favorite venue of McCain's during his campaigns in the state.

