Now he doesn't have to spell that out anymore - but he does have to spell out his record.
In the CNN debate Jan. 21, he said: "On issue after issue that is important to the American people, I haven't simply followed, I have led."
From votes for abortion rights to lessening penalties for marijuana use to raising doubts about capital punishment, Obama is a traditional liberal, CBS News correspondent Dean Reynolds reports.
"I can't think of a tax increase that he didn't embrace," said state Sen. Bill Brady, R-Ill.
Still, Brady considers Obama a friend who was able to work effectively with both parties in Springfield, pushing social welfare and justice legislation.
"Republican, Independent or Democrat, he was very willing to ask anyone for help on an initiative he may be pursuing," Brady said.
That includes a bill requiring police to videotape interrogations. Obama overcame opposition from the governor, the police and members of both parties to pass the bill.
But there was something else about his time in Springfield.
In more than 4,000 votes, Obama voted "present" - that's the yellow button on the right of a state Senate voting apparatus - some 129 times.
That's a cop-out, say his critics.
"That's not 'yes,' that's not 'no,'" said Sen. Hillary Clinton while debating Obama. "That's 'maybe.'"
Obama even voted "present" on a bill involving sexual abuse that he had sponsored himself - saying he discovered legal questions after its introduction.
And yet voting "present" in Illinois can be used to avoid making a choice.
"It's not that unusual for this to occur," said Chris Mooney, a political scientist at the University of Illinois.
His rise in the current campaign is consistent with what has to be considered a charmed political life.
"The hopes of a skinny kid with a funny name that America had a place for him too," Obama said in his speech at the Democratic Convention in 2004.
That speech brought even Hillary Clinton out of her seat. And his Senate race took off just as his Republican opponent fell apart.
"The Obama phenomenon. A wave that just can't be stopped … it just continues to crest," said David Mendell of the Chicago Tribune and author of the book, "Obama: From Promise to Power."
Once in Washington, Obama fought to cut dependence on foreign oil, provide relief for wounded soldiers and he led a successfull fight to limit the influence of lobbyists.
"A lot of the detail in terms of the disclosure provisions for lobbying really came from Obama," said Norman Ornstein of the American Enterprise Institute.
But there are some questions.
In a February debate, he said: "I said very early on I would not take PAC money. I would not take money from federal-registered lobbyists."
Not now - but he did accept at least $1.2 million from special interest political action committees for his U.S. Senate campaign. And that helped elect him.
He takes credit for battling the nuclear industry, but a plan to improve reporting of radiation leaks was watered down - by him - partly due to industry opposition. And it never passed.
Employees and officials of Exelon - one of the companies involved - contributed almost $270,000 to his presidential and Senate campaigns.
Obama is a member of the Senate Foreign Relations Committee - and but he's been absent a lot. He has yet to meet British Prime Minister Gordon Brown, for example, or the leaders of Germany, Russia or even Canada, according to his staff.
Because he is so new to the national scene, questions persist about Obama's identity - who he really is; questions that political opponents will be only too happy to answer in the fall.
No flag pin on his lapel? No hand on his heart that once? Opponents call it unpatriotic. Is he a Muslim? The whispering persists no matter how often Obama responds.
"I've been going to the same church for 20 years, praising Jesus," Obama said.
That church is the Trinity United Church of Christ. Self-described as "unashamedly black," with an emphasis on African culture, the church has been targeted by critics who call it separatist, racist and anti-Israel.
Obama has been a member for 20 years.
"I consistently have not only befriended the Jewish community, not only have I been strong on Israel, but, more importantly, I've been willing to speak out even when it is not comfortable," Obama said.
Obama has said the church's former pastor - and his spiritual mentor - Jeremiah Wright, is "like an old uncle who sometimes will say things that I don't agree with."
Among Wright's pronouncements: that "racism is how this country was founded and how this country is still run."
A church-related publication saluted Nation of Islam Minister Louis Farrakhan - a well-known anti-Semite, who in turn has praised Obama's candidacy as recently as last Sunday.
It's a gesture Obama rejected Tuesday night, after some prodding.
"There's no formal offer of help from Minister Farrakhan that would involve me rejecting it," he said. "But if the word 'reject' Sen. Clinton feels is stronger than the word 'denounce,' then I'm happy to concede the point, and I would reject and denounce."
Obama's long association with a now-indicted developer named Tony Rezko could also be a liability. An Obama fundraiser from the early '90s, Rezko goes on trial fraud next week. But his unsavory reputation was well known for years.
And it raised eyebrows when Obama and Rezko's wife, Rita, bought property next to each other on Chicago's south side on the very same day in 2005 - even though by then, Tony Rezko was under federal investigation.
No one has charged Obama with wrongdoing, something he has been quick to point out.
"Nobody has indicated that in any way ... was I connected with any of the things that he did," Obama said.
For good measure, Obama has given some $150,000 in Rezko-related campaign contributions to charity. But government watchdogs scratch their heads.
"So, what we're left with is a question of, you know, really, what was he thinking? The warning signs to stay away were very clear," said Cindy Canary of the Illinois Campaign for Political Reform.
And Obama didn't heed them?
"Sen. Obama was very slow to walk away from Tony Rezko," she said.
Could it come back to haunt him?
"If we have problems in this campaign, I suspect it's not going to be because of mistakes I've made in the past. I think it's going to be the mistakes that I make in the future," Obama said on 60 Minutes.
His opponents will be waiting.