"We like to criticize everybody, including the president," David Keene, chairman of the American Conservative Union, said last week at the Conservative Political Action Conference.

An undercurrent of discontent with President Bush's policies was evident at the nation's pre-eminent gathering of conservative activists. As Keene put it, "When you talk to Republicans everywhere, they'll say, 'We love the guy, but—'" But what?

But too much spending, for one thing. "The Bush version of conservatism tends to favor bigger, more powerful government, and that runs straight up against the core belief of most conservatives," Keene said. And here's the big surprise: so does Bush's Iraq policy. "Part of the base belief of conservatives is that people in Washington have neither the confidence nor the ability to tell the people of Peoria, Illinois, how to live their lives," Keene said. "It seems inconsistent to say, 'Well, we may not be able to do that, but we do know how to organize societies halfway across the globe.'"

Yet immigration is the issue that poses the most danger for Bush. Many conservatives are intensely uncomfortable with Bush's proposed guest-worker program, which they regard as amnesty for lawbreakers. That discomfort is shared by many Americans outside the conservative base. Concern over immigration is a populist issue. And that makes it particularly dangerous for Bush.

A January Gallup poll taken for USA Today and CNN asked Americans to rate Bush's handling of seven issues. The president's lowest rating? Immigration—his approval rating was only 25 percent. That was lower than his ratings on terrorism (52 percent approval), the aftermath of Hurricane Katrina (41 percent), the economy (39 percent), Iraq (39 percent), health care (31 percent), and corruption (28 percent).

According to Frank Sharry, executive director of the National Immigration Forum, "The big question, the elephant in the room, is the 11 million here illegally." In a CBS News poll taken last year, the public opposed by nearly 2-to-1 a guest-worker program that would allow illegal immigrants to apply for work permits.

Rep. Tom Tancredo, R-Colo., a featured speaker at CPAC, is threatening to run for president in 2008 to spotlight the immigration issue. At a rally against illegal immigration last week on the U.S. Capitol grounds, he declared, "If the president of the United States wanted to, he could secure the border tomorrow.... He has the power to do so. He has the ability to do so. He has the resources to do so. The unfortunate, dirty truth of the matter is, he has no desire to do so."

A protest movement emerges when a constituency thinks no one is addressing its concerns. In the 1960s, the anti-Vietnam War movement and the George Wallace movement didn't see a dime's worth of difference between Democrats and Republicans. Right now, according to Gallup, people who say the immigration issue is extremely important split their congressional vote between Democrats and Republicans—as if to say there's not a dime's worth of difference.

What happens to protest movements? Typically, a major party reaches out. By nominating George McGovern in 1972, Democrats reached out to the anti-war movement. Richard Nixon used his Southern strategy to fold Wallace voters into the Republican Party.

In his State of the Union speech last month, Bush reached out to anti-immigration voters when he said, "We must have stronger immigration enforcement and border protection." But he also said, "We hear claims that immigrants are somehow bad for the economy, even though this economy could not function without them.... We must have a rational, humane guest-worker program."

The immigration issue, Sharry says, creates "a civil war among Republicans as to whether they can be pro-enforcement and pro-immigrant, the way President Bush seems to want." The Democratic base is also split over immigration. But don't look for a civil war among Democrats. Democrats think the immigrant vote will eventually restore them to power. Democrats prefer to talk about outsourcing of jobs, a less-risky issue. It's not anti-immigrant. It's anti-business.