The compromise immigration bill is a valiant effort to end the impasse over one of the nation's most difficult issues. But it may well fail because — while it does address the main substantive issues — it fails to satisfy the political realities.

There's another problem: The Senate bill is so complicated it risks being picked to death in the same way our legislative system has proven unable in recent years to digest complex plans on healthcare and entitlement reform.

Still, credit should go to President Bush and the bipartisan panel of senators who worked out the proposal for trying. They looked past short-term political pressures and proposed something in the country's long-term interests. It's a sign of what could have been had Bush decided to work more closely on other issues with members of both parties.

In broad terms, the plan meets the basic substantive requirements by strengthening border enforcement, providing a temporary guest worker program and mapping a long-range path to citizenship for the 12 million here illegally.

But reaction to specific provisions reflects the same political concerns that previously prevented enactment of a new immigration plan:

Hard-core opposition to anything that can be portrayed as amnesty among rank-and-file Republicans is spurring some top GOP presidential candidates to oppose the compromise.

That's been the response of former Massachusetts Gov. Mitt Romney, former Arkansas Gov. Mike Huckabee and potential candidate Fred Thompson, the former Tennessee senator.

Only Arizona Sen. John McCain, a principal architect, backs it, and he promptly encountered conservative flak.

Former New York Mayor Rudy Giuliani, in the past considered pro-immigrant, delayed taking a position.

Several Democrats expressed concern about the revised guest-worker plan, including Gov. Bill Richardson of New Mexico, Sen. Barack Obama of Illinois and former Sen. John Edwards of North Carolina. New York Sen. Hillary Rodham Clinton withheld comment pending further study.

Bush, having done the right thing both substantively and politically, faces the realities of his diminished clout. He may find it extremely difficult, even impossible, to produce the 70 House GOP votes that Democrats say they need to pass a bill.

Indeed, it's unclear whether the votes are there to pass it in the Senate; even some who voted on May 21 to begin the debate oppose various provisions.

Underlying the jockeying is the question of whether the two parties would benefit more from a bill or a continued impasse.

Beyond internal party pressures to oppose anything that smacks of amnesty, many Republicans think they gain from hard-line anti-immigrant feelings — though the 2006 voting suggests they're wrong.

And many Democrats think that, even if they can't pass a bill, they will benefit from GOP antagonism toward ultimate legal status, especially among Hispanic voters in such states as Colorado, Arizona and Nevada.

The unfortunate thing is that, unlike entitlement and tax reform — where the pressures in future years may offer a better chance for action — this year may provide the best chance for immigration legislation, especially if Democrats win the White House next year.

Passage of any immigration measure requires support from both parties. It may prove easier to get the necessary Republican votes with the help of a GOP president.

Public opinion seems to favor passage of a comprehensive measure like the bill before the Senate, though survey results depend heavily on how the questions are posed.

A recent CNN poll showed strong public support for a program that allows illegal immigrants to apply for citizenship if they have a job and pay taxes. A Los Angeles Times/Bloom-berg poll showed more support for tougher enforcement of immigration laws and a temporary guest-worker program, rather than relying solely on tougher enforcement.

That suggests that politicians who back a balanced approach — for instance, Bush, McCain and many Democrats — are right about both substance and politics. But it doesn't guarantee this effort will succeed.

Carl P. Leubsdorf is Washington bureau chief of the Dallas Morning News. Reach him at cleubsdorf@dallasnews.com.