In announcing early Monday morning that he would seek reelection, President Obama launched a 20-month campaign that will aim to cast him as bringing about change while successfully managing crises at home and abroad.

Obama aides plan to define his tenure as a time of major progress, taking on issues such as health care that have long bedeviled presidents of both parties, improving the economy and winding down the war in Iraq.

In an e-mail to supporters, Obama said his campaign would be an effort “to protect the progress we’ve made — and make more.”

Obama faces a far different dynamic than he did in 2008, when he ran as an underdog outsider promising hope and change. Many who backed Obama then have soured after two years of difficult and, often, unpopular decisions on the economy, war, energy and a host of other issues.

“The good news is that he’s close to 50 percent, no opposition in the primaries, he can raise an unlimited amount of money, his base is pretty solid,” said Doug Schoen, a Democratic pollster who was a top adviser to President Clinton. “The bad news is independents have gotten away from him, the economic climate we are facing is pretty grim and we are fighting three wars.”

Obama must also reenergize the legions of supporters who propelled him to the White House but whose enthusiasm has dampened in the face of the compromises and setbacks of governing.

To that end, there were a number of echoes of Obama’s history-making 2008 campaign in his announcement. In a two-minute video included in his e-mail to supporters, for instance, a woman identified as Katherine from Colorado talks about “changes” followed by a young voter identified as Mike from New York who talks about “hope.”

“We’re not leaving it up to chance,” said a woman identified as Gladys from Nevada. “It’s an election that we have to win.”

The challenges Obama faces were made clear just hours after his announcement, when his administration said that Khalid Sheik Mohammed and four co-defendants accused of planning the Sept. 11, 2001, attacks will be tried in military commissions in Guantanamo. That’s a reversal from Obama’s campaign pledge to hold the trials in federal court and another reminder of the president’s unfulfilled promise to close the prison.

Obama did not appear in public on the day of his announcement, as aides said he would remain focused on governing. But in brief remarks to a group of Democratic activists in Portsmouth, N.H., Vice President Biden offered an economic theme that Democrats said will reflect the campaign’s message.

Biden reeled off a series of statistics that showed economic growth in the past several months despite Obama entering office during “the worst economic crisis since the Depression.”

“We stepped up,” Biden said.

Aside from rallying supporters, Obama launched his campaign Monday so he could start raising money for a race that may cost more than $1 billion. His announcement came at about the same time in his first term that Bill Clinton and George W. Bush announced that they were seeking reelection.

It also came before any of the prominent Republicans considering 2012 campaigns have formally announced that they are going to challenge him. Former Minnesota Gov. Tim Pawlenty, the only high-level Republican to officially form an exploratory committee, released his own video on Monday attacking Obama.

“I’ve got a question for you: How can America win the future when we’re losing the present?” he says in the spot, referring to a phrase repeatedly used by Obama.

Obama is unlikely to start personally taking on his potential opponents in the next few months. He will attend Democratic fundraisers, including one next week in Chicago, but will otherwise attempt to remain above the political fray.

His most obvious nod at his election prospects will be his schedule. He is giving speeches this week in the Philadelphia area and Indianapolis, the largest cities in two key swing states, a pattern that is likely to continue.

The president will face a much more difficult political map than in 2008. Three years ago, he won overwhelmingly in traditional swing states such as Pennsylvania and Wisconsin, gained narrow victories in states Bush had won such as Ohio and Florida and was the first Democrat in a generation to triumph in places such as Virginia, North Carolina and Indiana. His poll numbers have dipped in nearly all of those states, many of which elected Republicans to Congress, statehouses and state legislatures last November.

Most polls show Obama polling near 50 percent approval, making him a favorite to win but still in perilous territory, according to strategists in both parties.

Aware of his diminished standing, particularly among independent voters, Obama has been unofficially campaigning for reelection almost from the moment Democrats lost seats in November’s congressional elections.

Despite consternation from his liberal base, Obama has sought compromises with Republicans on key issues, allowing tax cuts for high-income Americans to stay in place while setting aside pushes from earlier in his term on more divisive issues such as immigration reform and climate change legislation.

Even as his recent shift to the center worries some Democrats, Obama has started appearing at party fundraisers and previewing his reelection message: progress and results. He lists achievements from the health care bill to repealing “don’t ask, don’t tell.”

“We have made extraordinary progress over these last two years. It’s been tough,” Obama said at a recent Democratic Party fundraiser in Washington.

He added, “But when you look back at the track record of work that we’ve done over the last two years, I think that it’s fair to say the promise that we made to the American people has been kept, that we have delivered on change.”

At the same time, he has hinted at public dissatisfaction with his tenure. In a recent interview with a television station in Denver, Obama said “I don’t think there’s a sense that I’ve been successful.”

In speeches in the past several weeks, the president has been delicately trying to tell voters the economy is improving, but still not seem out of touch as unemployment remains high.

“He can’t say it’s ‘morning in America,’ we have 8 percent unemployment,” said Steve Elmendorf, who was a top adviser to Sen. John Kerry’s (D-Mass.) 2004 presidential campaign, referring to Ronald Reagan’s famous phrase. “But he can say we are making progress.”

Staff writers Dan Balz, Chris Cillizza and Nia-Malika Henderson contributed to this report.

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