Four years ago, Barack Obama carried a promise of big change across Ohio to win this battleground state. Now, with just 12 days until the election, Mitt Romney is furiously trying to make up ground here by doing just that.

Over and over again Thursday — inside an industrial warehouse in Cincinnati, in a parking lot in Worthington and at a high school football stadium in Defiance — Romney promised to bring “big change” to the White House.

The Republican nominee effectively is co-opting Obama’s change message of 2008, casting himself as an energetic reformer while arguing that the president would continue leading the nation down a “status quo path.”

“The American people now have to recognize that given the big challenges and the big election we have, it’s time for a big change — and Paul Ryan and I represent a big change for America,” Romney told some 3,000 supporters in Worthington. “We’re finally going to tackle the problems politicians have spoken about for years but haven’t been willing to deal with.”

Romney kicked off two days of campaigning across Ohio, which has emerged as the central battleground, by starting to make his closing argument to voters here. He plans to deliver a major economic address Friday in Iowa that advisers said would more fully lay out what he sees as the choice American voters face.

“This is a critical time for our country, and the choice of paths we choose will have an enormous impact,” Romney said in Cincinnati. He talked about the nation’s poor-performing schools, rising debt and stubborn joblessness. “These challenges are big challenges. This election is, therefore, a big choice. And America wants to see big changes, and we’re gonna bring big changes to get America stronger again.”

The big change Romney is prescribing to put the nation back on course is of the conservative economic variety — lowering taxes, loosening regulations, reining in deficit spending, overhauling Medicare and shrinking the size and scope of the federal government. He infused his pitch with a spirit of bipartisanship as he tries to soften his image with independent voters, particularly women.

Although Romney has gained in recent weeks in state and national polls, he still lags slightly behind Obama in most Ohio polls. The Democrats appear to have an edge so far in early voting, although Romney is banking on a higher turnout on Election Day. Two senior Romney aides published a memo Thursday claiming a “steady upward trajectory” for Romney and arguing that the state was “unmistakably moving in Mitt Romney’s direction.”

Underscoring the urgency of winning Ohio, Romney spent the whole day here Thursday after two days of hopping around a slew of other swing states. After stopping by a Cincinnati diner in the morning (he ordered an omelet, potatoes and grits for $7.99), Romney traveled about 300 miles by bus.

Romney tried to build enthusiasm among his supporters and win over the few voters who remain undecided by projecting optimism. Sen. Rob Portman (R-Ohio) introduced Romney on the stump, saying he would “bring back the hope.”

Romney told his crowd in Worthington, “I want you to know how optimistic I am. This is about to get real good. . . . On November 7th, it’ll get a lot better with voting for big change!”

Obama’s campaign countered that the change Romney promises is a return to the “failed policies” of the Bush administration that led to the financial collapse. Obama spokeswoman Lis Smith charged that Romney would be a “rubber stamp for the far right wing,” citing his refusal to withdraw support for Indiana Senate candidate Richard Mourdock after the latter asserted that pregnancy after a rape was “something God intended to happen.”

But rape and Mourdock are two themes Romney has no interest in talking about; at several stops Thursday, he declined to answer questions from reporters about it. Instead, Romney focused on what he calls the “big choice” in this election.

“It’s time for a big change — to draw on the quality of the American heart, to come together, Republicans and Democrats, to finally reach across the partisan divide and reach a place where we come together for the American people,” the former Massachusetts governor said.

Romney repeatedly attacked Obama, belittling him for focusing on what he said were “smaller and smaller things” — though his rhetoric has not been as harsh as a year ago, when he launched his campaign saying the president had “failed,” or even a few months ago. Romney said that it’s the “status quo path,” not the president himself, that doesn’t know how to jump-start the long-beleaguered economy.

Voters at Romney’s events responded enthusiastically to his promise of change. In Cincinnati, the throng of 3,000 was electric. Every time Romney spoke of “big change” — a phrase he used about a dozen times — his supporters’ screams were nearly deafening. And in Worthington, Ellen Bell, a homemaker in her 50’s, said of Romney the change agent, “Bring it on!”

“We need change. I need hope,” said Paul Cross, 56, who owns a sporting goods store in Columbus. “In the last 18 months my sales are down over 33 percent. I had eight employees and I’m down to three. The parking lot of the mall my store is in is empty. I need someone that can get money into the economy again. It is about jobs. We all know it. We’ve got to have change.”