Barack Obama appears ready to get more aggressive in the race for the Democratic nomination — saying that the time has come for him to make distinctions between himself and frontrunner Hillary Clinton.

In the interview with The New York Times that appeared in Sunday's edition, while en route to a campaign stop in Ohio, Obama accused Clinton of "trying to sound or vote like Republicans, when it comes to national security issues," and of "kicking the can down the road" on Social Security.

Click here to read The New York Times article.

"If Hillary Clinton is the nominee, then we have a repetition of 2000 and 2004," he said.

Asked whether Clinton has been truthful about what she’ll do as president, Obama answered "no," saying "on Social Security, in the last debate, she was very explicit about not wanting to disclose how she would approach it. On Iraq and Iran, I think there has been a tendency to go back and forth in her positions."

Obama added, "It’s been very deft politically, but one of the things that I firmly believe is that we’ve got to be clear with the American people."

Critics and supporters alike have noted Obama’s unwillingness to criticize Clinton openly. But that has changed in recent weeks. The underdog candidate has regularly referred to Clinton by name, indicating as early as Oct. 12 that he was going to go directly for Clinton.

"We’ve spent a lot of time laying the groundwork in terms of biography, and now as the leaves turn and the air gets cooler, there’s going to be more of an emphasis on the contrast between myself and the other candidates in the race," he said.

Since mid-month, Obama has pointed out differences in their views on the Iraq war, policy, health care reform and has taken the New York senator to task as a Washington insider.

Obama now calls Clinton the "default" candidate, saying she is the "brand name" in Democratic politics. He has used words like "ambition" and the ever-famous Bill Clinton strategy fo "triangulation" to describe what’s wrong with Washington politics, and most recently, Obama told Iowa voters that Clinton has evaded answering questions on Social Security

"You’re not ready to lead if you can’t tell us where you’re going," he said.

If the new approach is any indication, Obama may charge headlong into Clinton at Tuesday’s Democratic debate in Philadelphia. The campaign has been mum on Obama’s debate strategy, but considering the campaign contacted The New York Times "to signal a change of course" just days before the candidates gather in Philadelphia, it seems a safe bet that Obama will come out swinging.

Obama has said he always planned to ramp up his arguments against Clinton at the end of the year.

"Our strategy has always been based on making the case to voters when they were paying attention and that is in late October, November, December," he said, adding that he first wanted to emphasize his biography before making distinctions between himself and the other candidates.

"Now’s the time for us to make these distinctions clear," he said. "But what I will continue to resist is the traditional perspective that we should be ginning up fights or trying to knee-cap the frontrunner because I don’t think that is what the country is looking for either."

Obama's estimation may be correct. Iowa voter Kathy Powell told FOX News that while she thinks Obama may have to go negative to beat Clinton, she also thinks "it's really important for Democrats to have a really good chance to win, and I don't want to see a lot of infighting. I've seen that before."

The Clinton campaign immediately posted The New York Times interview at the top of its news Web site. The campaign’s stock response to any Obama attack has been to accuse the Illinois senator of abandoning the "politics of hope."

Obama Communications Director Rob Gibbs surmised that the Clinton campaign's own "politics of hope" meant hoping that nobody asked any questions.

"She started this campaign not wanting to have a conversation and in Iowa it's going to be a conversation that involves more than just her. And that's what we're going to do — we're going to talk about the differences and clearly we think Senator Obama has a better vision and a better plan on how to run this country and that's what we're going to talk about," Gibbs told FOX News.

Obama said he was "amused" by his opponent's critique, calling it "silly."

"The notion that somehow changing tone means simply that we let them say whatever they want to say or that there are no disagreements and that we’re all holding hands and singing ‘Kumbaya’ is obviously not what I had in mind and not how I function," he said. "Hope is not ignoring differences or ignoring problems."

The attacks are likely to escalate on both sides as the Iowa caucuses approach. But Obama insisted he won’t make it personal. "I am not interested in tearing into Hillary Clinton, just for the sake of tearing into her," he said. "I think she is an admirable person, I think she’s a capable senator.

"The case I’m making is not that she’s a terrible person or would be a terrible president, the case I’m making is that I would be better at those things that the country needs right now."

FOX News' Bonney Kapp contributed to this report.