Pennsylvania Republican Sen. Rick Santorum and his Democratic challenger, state Treasurer Bob Casey Jr., are at odds on the volatile issue of immigration.

The incumbent opposed and sharply criticized the bill passed this week in the Senate, while Mr. Casey said that, despite some reservations, he would have supported the measure.

Within hours of Thursday's bipartisan vote approving the bill, each campaign had issued statements criticizing the other's stand on an issue that promises to be among the key dividing lines in their closely watched race.

Mr. Santorum favors an approach closer to the enforcement-oriented version the House passed in December, one that lawmakers are expected to have difficulty reconciling with the Senate bill's more sweeping changes. Mr. Santorum, the Senate's third-ranking Republican, voted with a majority of his GOP colleagues in opposition, parting ways with other powerful Republicans such as President Bush, Senate Majority Leader Bill Frist, R-Tenn., and Sen. John McCain, R-Ariz., a key architects of the Senate's version.

Mr. Santorum denounced the measure as "a broad amnesty program for most illegal aliens," a characterization Mr. Casey and Senate proponents disputed.

Mr. Casey's support for the bill was his most specific stand on immigration legislation to date. After its passage, he issued this statement: "The immigration bill is not perfect, but it is a consensus bill that drew the support of Republicans and Democrats. ... If I were in the Senate, I would have voted for [it] because it is long past time to take action, and this bill contains important reforms."

The Senate bill, approved 62-36, would tighten border security, establish a guest worker program and create an opportunity for many illegal immigrants in this country to become citizens.

In a commentary posted on his campaign web site, Mr. Santorum contended that the measure would reward illegal behavior while imposing significant costs for social services on governments at every level. He also argued, "The low hurdles to citizenship this bill erects -- making illegal immigrants stand at the back of the immigration line while remaining in our country ... -- mock and demean the sacrifices of those who waited years to immigrate through our established legal channels."

Mr. Casey disputed that analysis on his web site: "Let me be clear, this bill is no amnesty as my opponent claims."

He cited a defense of the bill offered by Sen. Arlen Specter, R-Pa., who noted on the Senate floor that it imposes on illegal aliens who seek citizenship these requirements: payment of fines, repayment of three years in back taxes and demonstrated English-language proficiency in addition to a criminal background check.

While Mr. Santorum broke with a White House that favors key elements of the Senate bill, Mr. Casey lumped them together in a parting shot that contended, "The fact that we're only now addressing border security in an election year is a sign of their failure and proof that we need a new direction in Washington."

The Senate bill, product of a delicate series of compromises, has critics on the left and right. Its declaration of English as the national language and provision for deporting millions of aliens has drawn fire from civil libertarians. On practical grounds, some have argued that the scale of deportation it envisions, while much less than that of the tougher House bill, is simply unrealistic. And the paths to citizenship it outlines have drawn particularly sharp attacks from the right.

There would appear to be little risk for Mr. Santorum in distancing himself from a president with slumping popularity.

If Mr. Santorum had supported the bill, it would have sparked a firestorm among many conservative activists whose grassroots energy is crucial to his hopes for moving ahead of a challenger who has consistently led him in the polls.

Immigration joins economic issues that have offered the strongest contrasts between the two contenders. Mr. Casey has criticized his rival's record on health care, taxes and the deficit.

Mr. Casey also has faulted Mr. Santorum for not raising question about the administration's policy in Iraq, but, in contrast to some other Democrats, he has not offered a clear alternative, such as the call by Pennsylvania's Rep. John Murtha, D-Johnstown, for an early troop withdrawal.

The two men's contrasts on hot-button social issues such as gun control and abortion have been relatively muted. But as the House and Senate seek an immigration compromise, the issue offers a distinct policy difference between the two -- one sure to produce a sharp visceral reaction among many voters.