PORTLAND, Ore. - President Obama went on the defensive today over his understanding of small business owners, saying Republicans "may have tipped a little bit over their skis" in their attempts to use his words against him.

"Earlier today Gov. Romney was at it again. He's been twisting my words around to suggest I don't value small business," Obama said of the presumptive GOP presidential nominee at a campaign fundraiser inside the Portland Convention Center, the second time in as many days he has hit back at the Republican attacks.

"In politics you have to endure certain amount of spin. Everybody does it; I understand it. Those are the games that are played in campaigns," he said. "Although I have to say, when people omit entire sentences from a speech and they start splicing and dicing, they may have tipped a little bit over their skis."

The president was referring to a new Romney TV ad and statements by the candidate and his surrogates that have taken out of context a line from one of Obama's speeches.

Republicans say Obama denigrated the success of small business owners on July 13 when he said, "If you've got a business, you didn't build that."

As their attacks have grown in volume, the president's campaign has begun to more aggressively and defensively point out that the "that" to which Obama was referring was the American infrastructure system.

"If you were successful, somebody along the line gave you some help. There was a great teacher somewhere in your life. Somebody helped to create this unbelievable American system that we have that allowed you to thrive. Somebody invested in roads and bridges. If you've got a business. you didn't build that. Somebody else made that happen. The Internet didn't get invented on its own. Government research created the Internet so that all the companies could make money off the Internet," Obama said in Roanoke, Va.

Romney and his campaign insist even the spirit of what Obama said shows a fundamental misunderstanding of the economy.

"President Obama said that business owners 'didn't build' their companies, and he meant it," Romney campaign spokesman Ryan Williams said in a statement to ABC News. "People across this country agree that government isn't responsible for building our nation's businesses. It's just the latest detached remark from an out of touch president who has consistently made life more difficult for job creators and middle-class workers."

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The president's rebuttal to Romney on the issue of small businesses for a second straight day suggests heightened concern among Democrats that negative Republican portrayals of Obama might be taking hold.

"I believe with all my heart that it's the drive and ingenuity of Americans who start businesses that lead to their success," Obama said, testifying to his support for small business owners before a friendly crowd. "I believe in the ability for someone who's willing to work hard and turn idea into a profitable business, that's what makes us such a robust economy."

And as he did in a new campaign TV ad released earlier today, the president also tried to clarify his view on the role of government investment in public services and infrastructure as part of a plan to help businesses succeed.

"The idea that what it takes to give our people and businesses best possible chance at success involves individual initiative," he said, "But also us as a nation working together to create a platform for success.

"Mr. Romney disagrees with this, and he's entitled to his opinion. But the approach that he's talking about is not going to help small businesses and it's not going to create more markets for large businesses. He is wrong," Obama said. "We did not build this country on our own, we built it together. And if Mr. Romney doesn't understand that then he doesn't understand what it takes to grow this economy in the 21st century."