WASHINGTON -- The Senate's budget-cutting marathon last week was a perilous time for all 100 senators who must answer to the varied interests of their constituents, but few senators' votes were watched as closely as those of Sen. Rick Santorum, R-Pa., who faces re-election next year and is trailing his chief opponent in the polls.

Sen. Rick Santorum

The Senate's rapid-fire series of votes to scale back mandatory spending programs like Medicare and Medicaid -- the first effort of its kind in eight years -- presented difficult choices for a senator from a state with the second-oldest population who has cast himself as both a proponent of fiscal discipline and a compassionate conservative.

Mr. Santorum, who said members of Congress have gotten "the message from across America that they want to see fiscal responsibility," was one of 52 senators supporting the Senate's $35 billion in net spending reductions over five years. But he may face more difficult decisions ahead when the Senate's proposal is reconciled with the House's $53.9 billion version, which could come up for a full House vote as early as this week. Lawmakers from both chambers will vote on the final package after the two versions are meshed in a joint panel.

That package may include many of the less politically palatable cuts that are in the current House version -- including an $844 million reduction in food stamps that cuts off 295,000 people by tightening eligibility, a $577 reduction in aid for families who take in foster children, and a $4.9 billion slice from programs that track down parents who skip out on child support.

If Mr. Santorum votes for a final package with those kinds of cuts, his chief challenger, state Treasurer Robert P. Casey Jr., is all but certain to make them an issue in the campaign because Mr. Casey believes they illustrate what he considers the Republicans' misguided priorities.

Complicating matters for Republicans concerned about the deficit is the fact that the Senate's $35 billion spending cuts could be negated by the upcoming Republican proposal to extend the president's tax cuts, estimated to take $70 billion from the Treasury over five years.

Though Mr. Santorum expressed uncertainty earlier this fall about whether it was prudent to move forward with the entire tax package after the $62.5 billion allocation to help Hurricane Katrina victims, he said last week that he supported the package in its entirety.

Democrats are fighting against provisions Mr. Santorum supports such as the $21 billion extension of the capital gains and dividends tax cut, which primarily benefits those making more than $200,000 a year. A Democratic analysis of 2003 Internal Revenue Service data showed that Pennsylvania taxpayers making between $30,000 and $50,000 got about $5 in savings with the capital gains tax cut, while those making more than $200,000 saved $3,632, on average.

But Santorum said the extension of the president's tax cuts are needed for stability in the tax code and to strengthen the economy.

"I don't think most folks want taxes to go up at this time," he said. "From the standpoint of predictability, it's important to have these provisions extended."

That is a view disputed by Mr. Casey, who said the Bush administration has been "reckless fiscally" and described Mr. Santorum's position on tax cuts as irresponsible.

"When you're taking a record surplus and turning it into a record deficit, you make the situation worse [by] supporting tax cuts for the top 1 percent," Mr. Casey said in a telephone interview on Thursday. Mr. Santorum "continues to assert that he's a senator who votes to cut spending and votes responsibly. I think the evidence is just the contrary," he said.

"In 2006, we're going to have a debate about this, because it's about time we put the fiscal house of the federal government in order."

Mr. Santorum first made his mark as a fiscal conservative in the House, where he received high ratings from organizations such as the American Conservative Union, which monitors spending issues. And in his first Senate term, Mr. Santorum gained notoriety when he appeared in the Senate chamber bearing a chart that said "Where's Bill?" to protest President Bill Clinton's failure to present a balanced budget.

He showed that inclination toward fiscal restraint during Thursday's spending debate when he joined 31 senators in support of a measure that would have frozen spending programs except for defense at current levels beginning in 2007. The measure ultimately failed and was not supported by Republican moderates, including Sen. Arlen Specter, R-Pa..

Earlier in the process, Mr. Santorum also supported the 2.5 percent cut in agriculture subsidies to farmers, which amounted to a savings of $1.3 billion over five years.

But Mr. Santorum also voted against an effort supported by Democrats and six Republicans to reinstate the so-called pay-as-you-go rules, which would have required tax breaks and most new spending to be matched with spending cuts or tax increases.

Mr. Casey criticized Mr. Santorum's votes on both measures and said he would have supported the pay-as-you-go measure, but not the spending freeze because he believes it would jeopardize the quality of homeland security and veterans' health care.

A number of votes on health care last week are also likely to come up in next year's campaign.

Mr. Santorum voted against a measure that would have delayed an increase in Medicare premiums for seniors, as well as a provision that would have allowed the secretary of health and human services to negotiate directly with pharmaceutical companies to lower the price of medicine under the new Medicare prescription drug benefit.

Mr. Santorum opposed putting off the Medicare premium increase saying it was caused by a 1 percent increase in payments to physicians covered by the program; he is concerned if the physicians do not get the increase fewer doctors will participate in the program. He has opposed the measure that would have permitted government to negotiate lower drug prices for the Medicare prescription drug benefit saying he believes the government would be meddling with market forces.

Mr. Santorum said in an interview last week that he tries to strike the balance between his deficit reduction goals and spending programs popular with his constituents.

One example in budget negotiations in the agriculture committee this year was when he insisted on preserving a two-year extension of a $998 million program targeted at small dairy farmers that benefits many producers in Pennsylvania.

He said he appreciates the more ambitious scope of cuts that the House has tried to achieve and said he may favor their broader spending cuts.

"I think it would be great if we could come up with a package that's bigger. I'm not at all opposed to doing more deficit reduction," Mr. Santorum said. But he said, "I'm going to do my best to defend Pennsylvania."