AMES, Iowa — Mitt Romney stepped up his bid to strip President Barack Obama of the popular attributes that propelled his candidacy four years and claim them as his own, casting himself in a formal speech here as an agent of change who would govern as a post-partisan president.

Saying the nation is at a critical juncture, the Republican nominee repeated his newest campaign mantra - that he would usher in "real change" - and summoned Americans to shift course to meet the challenges the country faces.

But as Romney insisted his campaign was on the march — and aides tried to expand the playing field by arguing Minnesota suddenly was competitive — the electoral math lingered as a serious obstacle for the Republican. Fresh polling in the critical battleground of Ohio suggested Obama still holds a slim lead there, and his state campaign chairman acknowledged Romney's difficulty of taking the White House without Ohio.