When President Bush traveled to Georgia Tuesday to promote the Senate immigration bill, he didn’t sound like he was happy with the way the debate was going. “If you want to scare the American people, what you say is the bill’s an amnesty bill,” he complained. “That’s empty political rhetoric trying to scare our citizens.”

And that’s not the rhetoric of a politician who feels like he is currently winning a policy battle. This debate wasn’t supposed to get so heated. When a bipartisan group of senators announced the immigration deal two weeks ago, Democrats and Republicans alike were predicting swift passage, perhaps before Memorial Day weekend. What gives?

We’ve been down this road before. When the Democrats took control of Congress, it was widely assumed that President Bush would finally get the path to citizenship for illegal immigrants that his own party had denied him. But when Congress convened this year, there were reports that key Republican senators — including a couple of presidential candidates — were backtracking on an immigration partnership with Ted Kennedy.

Then Congressmen Jeff Flake and Luis Gutierrez introduced their bipartisan bill to a flurry of favorable editorials. Yet instead of blossoming into the fruitful immigration compromise Washington editorialists hoped for, the legislation quietly withered. Whenever a bipartisan consensus on this deeply controversial issue seems close at hand, fissures appear and ultimately nothing much happens.

It isn’t just the restrictionist right, although that’s the group the president is targeting when he insists the current immigration bill is no amnesty. Most of the leaders in both parties want to give legal status to millions of illegal aliens. But their specific concerns aren’t identical and the fragile coalition behind the Senate plan is threatened by its own internal contradictions.

The bill’s Republican supporters see immigration primarily as a labor problem. They want to open legal paths for willing workers to satisfy employers’ cheap labor needs. Where the Republicans see workers, the Democrats see voters. As a result, the Kennedy crowd wants a generous path to citizenship, presumably in the hope that it will satiate their demand for new Democratic voters.

The Senate immigration deal was an attempt to split the difference. With immediate probationary status, a fairly liberal Z visa program, and citizenship opportunities for illegal immigrants, the Democrats get the amnesty they want. In return, the bill’s Republican backers get a guest-worker program tailored to their specifications plus an eight-year shift away from family reunification toward a point system that favors higher-skilled workers.

Despite potential killer amendments from both sides, the bill so far seems to be holding together a solid bipartisan majority in the Senate (although we don’t know for sure — the anti-amnesty backlash derailed plans for a rapid vote). But with a few sharp tugs to the right or left, the whole coalition could unravel.

The bill is already being hit with a tough left-right combo in the press. Conservative radio king Rush Limbaugh has labeled it the “Destroy the Republican Party” Act. The New York Times editorial page doesn’t like it much better, because the path to citizenship “is strewn with cruel conditions, including a fine — $5,000 — that’s too steep and hurdles that are needlessly high.”

This unusual dynamic may also play out in the House. Many of the chamber’s Republicans remain committed to the enforcement-first position. Minority Leader John Boehner has famously called the Senate bill a “piece of s–t.” But not every critic can fairly be described as an immigration hardliner.

Rep. Zoe Lofgren, the California Democrat who chairs the House Judiciary Committee’s immigration subcommittee, has criticized the move away from “promot[ing] family values with family reunification.” Many black and Hispanic congressmen have objected to the point system and the guest-worker program, two provisions designed specifically to win Republican votes. “They are asking that people coming into the country come in speaking English, skilled, educated and fluent — well, that is going to be a real hurdle for people who come from very poor, underdeveloped countries,” Rep. Diane Watson, another California Democrat, told the Washington Times.

If the Senate bill ends up causing the Congressional Hispanic Caucus to join forces with Tom Tancredo’s Congressional Immigration Reform Caucus against the Chamber of Commerce, it will be a tough fight.

Some senators are making the fight more likely. While Republicans have so far failed to strengthen the border-security triggers or the requirements for the Z visas, a few get-tough amendments have passed. One is Sen. Lindsey Graham’s proposed mandatory minimum sentences for certain illegal border-crossers. There are also get-soft amendments being batted around, most notably an attempt by Democratic frontrunners Hillary Clinton and Barack Obama to effectively delete the point system from the bill.

In the end, all the details that allow Bush, Kennedy, and company to abjure the amnesty label keep dooming these bipartisan immigration pacts. This year there are many political factors that favor the “comprehensive” approach. The cleverness of its proponents just isn’t one of them.