U.S. Rep. Ron Paul, R-Texas, speaks during a news conference Tuesday, April 26, 2011, in Des Moines, Iowa. Paul says he's forming a campaign exploratory committee as he moves closer to again seeking the Republican nomination for president. (AP Photo/Charlie Neibergall) (Charlie Neibergall/AP)

More clarity to the 2012 presidential campaign came this week as Rep. Ron Paul (R-Tex.) announced the formation of an exploratory committee and Mississippi Gov. Haley Barbour (R) decided to abandon a run. The latter move prompted questions about the rest of the field — notably, whether Barbour’s good friend Indiana Gov. Mitch Daniels (R) would now get in the race.

Even so, real estate mogul Donald Trump again dominated the news cycle, as President Obama decided to release his long-form birth certificate, an issue that has been front and center in Trump’s exploration of a possible bid. During an eight-hour trip to New Hampshire, Trump met with GOP party leaders and activists and declared victory for forcing the president’s hand.

Obama, meanwhile, continued his push to raise money among wealthy Democratic donors in New York and stopped by Oprah Winfrey’s show with his wife — the episode airs Monday. Eyeing Hispanics, he discussed immigration at the White House with prominent Latinos, and traveled to Florida to address graduates of Miami Dade College, which has a majority-Latino student body.

Here’s what else happened this week:

Romney: The Anti-Trump? The contrast couldn’t have been clearer. Mitt Romney showed up at a gas station in Manchester, N.H., in jeans, an open-collared shirt and riding in a silver Ford Escape. Two days earlier, Trump landed in the Granite State in a chopper, his red tie flapping in the wind, and tooled around in a black stretch limousine. Romney has said he welcomes a Trump candidacy, while Trump has bragged that his net worth is bigger than Romney’s, making him more qualified to be president.

New Hampshire strategists said that Trump will have to lose the limo to play in the first primary state, and he just might do that in his visit next month. Until then, Romney has the edge with the sort of retail politics people in New Hampshire have come to expect. In Trump he has a clear foil, and an already combative one at that.

Barbour’s lessons. Every GOP politician considering a 2012 run likely looked closely at Barbour’s decision to pull the plug. Barbour is considered by leaders of both parties to be a top-notch political strategist, but he was in the cellar in national and regional polls and never found a constituency, much less a plausible path to victory. And no matter Obama’s flagging approval ratings, history shows that it’s awfully hard to beat an incumbent president, given his inherent advantages.

Barbour weighed all of this in his decision. Not only that, the exposure that comes with an unsuccessful campaign can have a great downside. Witness Rudy Giuliani, who stumbled badly in his 2008 run, tarnishing his standing in his party. And then consider reiired Gen. Colin Powell, who still shows up in some polls as a credible contender and has the political and cultural capital of a man who could have been president.

Slow Walk to South Carolina. How many possible GOP contenders will show up Thursday in Greenville, S.C. for the first 2012 debate? Former Minnesota governor Tim Pawlenty will definitely be on stage, and he subtly knocked his potential rivals who have yet to commit, saying in a statement, “It’s important that Republicans show up now, talk about their records, and begin the debate on how we best can defeat this president.”

Debate organizers pushed back the April 29 RSVP deadline to give potential candidates more time. Yet the stage could still be filled with second-tier contenders — an image that the White House, eager to draw contrasts, would likely welcome.

But the dance over the debate raises the question of how long GOP contenders can continue to unofficially run for president, thus ceding the spotlight to Obama and, to a certain extent, Trump.