WASHINGTON – Liberal activists are blasting Barack Obama for shifting toward the center on a variety of issues, creating a tension that poses risks for his candidacy. But in another sign of how the Internet is changing politics, the conflict has led to a surprisingly direct give-and-take between candidate and critics – a development that may prove healthy for the campaign.

Just as Obama mastered Internet politics as he swept toward the nomination, now activists and bloggers are using some of the same tools to bring pressure to bear on the candidate. Obama, a former community organizer, is responding in kind – and says he’s happy to be held accountable.

“Just receiving and listening isn’t enough online,” said Zephyr Teachout, an online pioneer for the 2004 Howard Dean campaign who participated Friday in Netroots Nation, a gathering of activists and bloggers in Austin. “People will push back, talk back, and the Internet makes that easier.”

The debate focuses on Iraq, wiretapping and whether the Illinois senator is moving away from progressive principles as he begins a high-profile foreign trip and prepares for the general election. Some activists warn Obama could ultimately face a drop-off in enthusiasm and donations from his Democratic base – and his moves already are fueling significant opposition.

Case in point: More than 24,000 people have joined a group on my.barackobama.com, a social-networking site set up by the campaign, that implored Obama to oppose a compromise on a major surveillance bill.

Instead, Obama voted for the bill, which effectively grants immunity to telecommunications companies involved in the Bush administration’s warrantless wiretapping program. He had threatened to filibuster that provision a few months ago.

Some supporters on the Web site chastised Obama for turning his back on the Constitution. Some wanted their donations back. Markos Moulitsas, founder of the influential Daily Kos blog, said he would withhold contributions because Obama was “betraying his claims of being a new kind of politician.”

Then, as he prepared to leave for an overseas trip, Obama signaled some flexibility on his timetable for removing combat troops from Iraq, saying he would “refine” his position after talking to ground commanders.

In recent weeks, Obama also said he supported gun rights, the death penalty in some non-murder cases and his own version of President Bush’s faith-based initiatives. In a speech on patriotism, he chided MoveOn.org, a group that helped him secure the nomination, for its “Gen. Betray Us” ad targeting Gen. David Petraeus.

Some liberals seethed. Some were surprised that Obama “was acting like a politician, doing what he thinks he needs to do to get elected,” said Rick Jacobs, founder of the Courage Campaign, a grass-roots progressive group in California.

Most activists will vote for him, “but it dims the enthusiasm, and puts a damper on his fundraising base,” said Jacobs, who was “very disappointed” by Obama’s reversal on surveillance.

Customary shift

Seeking the center during the general election is a time-honored quest for presidential candidates, and John McCain is trying to walk the same line. McCain, in fact, may face a deeper lack of enthusiasm in his party among conservatives who have clashed with him over the years.

Obama, still not well-known to many voters, has to be wary of being pigeonholed as a conventional liberal, said Steve Schier, a political scientist at Carleton College in Minnesota. And if he shifts too much or too quickly, “then he runs the risk of being seen as an opportunist and flip-flopper,” he added.

Independents who don’t see the world through a left-right prism may be uneasy if they are unsure of Obama’s core principles. Activist and blogger Arianna Huffington made a similar point: “My problem isn’t that Obama doesn’t always agree with me. My problem is that Obama has started to not always agree with himself – falling prey to the conventional wisdom sirens.”

But Obama moved quickly to respond to the criticism – and he addressed his critics directly, on the Internet. He posted a lengthy explanation of his surveillance vote on my.barackobama.com, telling angry backers that “I’m happy to take my lumps.” He added, “I’m certainly not perfect, and expect to be held accountable too.”

He also gave a major speech on Iraq and broader security issues, renewing his pledge to “safely redeploy combat forces” from Iraq in 16 months, and send additional forces to Afghanistan. “It’s time to end this war,” Obama wrote about Iraq in a New York Times opinion article Monday.

Jacobs said Obama saw the backlash to his surveillance vote “and needed to get back on track on Iraq.”

Teachout said the sharp criticism from thousands of Obama backers on the campaign’s own Web site is a sign of strength “that will puncture the idea that anything but abject loyalty hurts your campaign.”

Activists’ choices

Barbara O’Connor, a longtime observer of California politics and a Democrat, said Democratic groups are coming around to Obama, including feminist activists who backed Hillary Clinton.

“All those bloggers from California – where are they going to go, vote for McCain?” asked O’Connor, who heads the Institute for the Study of Politics and the Media at California State University-Sacramento.

O’Connor points to a Field Poll, released Wednesday, that shows Obama with a 24-point lead in the state, and the backing of 80 percent of Clinton supporters (8 percent favored McCain). Three times as many Obama voters (51 percent) as McCain voters (17 percent) said they were “very enthusiastic” about their candidate.

As of June, there has been no drop-off in donations. The Obama campaign took in $52 million last month.

There’s another factor that animates some of the tension between activists and Obama. Liberal blogs tend to be confrontational. Their success as an organizing tool has been fueled by anger over the Bush administration.

Obama’s tone and approach to politics is inclusive, stressing cooperation beyond what he calls “caricatures of left and right.”

“Listen to Obama, and you hear a bit of Arnold,” said O’Connor, referring to Gov. Arnold Schwarzenegger’s pleas for “post-partisan” solutions. “I think that’s what Obama really is – post-partisan.”