CINCINNATI -- Ohio's labor unions are mobilizing a massive campaign to help President Obama win the battleground state in November, providing volunteers to send thousands of mailers, work the phones and knock on doors.

"Labor is going to be energized and extremely active in re-electing Barack Obama," said Tim Burga, president of the AFL-CIO's Ohio chapter. "On the front porch, at the work site, on the phone and through the mail, it will be a massive ground game. We know we can't outspend the other side, but we'll be able to outwork them."

More than 700,000 union workers make up about 15 percent of Ohio's total workforce, the third-highest concentration of union muscle among nine swing states, according to the Bureau of Labor Statistics. The unions representing those workers not only will provide millions in campaign contributions, but thousands of volunteers who will help ensure Obama's supporters get to the polls.

Unions were wary of Obama in 2008, but eventually went to work for him. This year they're energized, particularly in Ohio, by Obama's support for an auto industry bailout and his defense of collective bargaining rights for public employee unions. That will give Obama a significant organizational advantage over Republican Mitt Romney in a state Romney must win. No Republican has won the White House without winning Ohio.

"Ohio auto workers are leading the recovery, with autos supporting one out of every eight Ohio jobs and workers in 80 counties," says a new Obama ad running in Ohio. "That's why President Obama stepped up, rescued the jobs, saved the industry. What would Mitt Romney have done?"

Romney hasn't given up hope of winning a piece of the union vote in Ohio. To bolster his standing among blue-collar workers, Romney will emphasize that while the president pledged his support for organized labor, he has fallen short in his promises to union workers, according to a Romney campaign memo.

"Obama has repeatedly proclaimed his support for unions ... he even promised to walk the picket lines with unions," the memo reads. "But Obama has sat on the sidelines at every opportunity."

Obama has failed to back the union in the current strike by public school teachers in his hometown of Chicago, the memo notes, and he was not engaged in the fight over public employees' collective bargaining power in Wisconsin.

Union leaders said they wished Obama had taken a stronger stance in the Wisconsin dispute. They were also disappointed more recently by his decision to hold last week's Democratic National Convention in North Carolina, a right-to-work state, and held their own convention in Pennsylvania rather than attend the Democratic gathering.

But Obama is overcoming those disappointments, at least in part because of the success of the auto bailout, an initiative that Romney opposed but which Obama claimed saved 150,000 Ohio jobs.

"President Obama showed courage and support in the auto industry rescue against some folks who were giving him counsel to not save the industry," Burga said. "What that means in Ohio is that General Motors and Chrysler have recovered and had great years.

"When you look at the records and the statements of the two candidates," Burga said, "there's no question that working families and labor are clearly coming out to support Barack Obama."