WASHINGTON — From farmers showing livestock at Harrisburg to schoolchildren in Erie, Republican Sen. Rick Santorum of Pennsylvania has lately fielded an onslaught of questions - some hostile - on the Senate impeachment trial of President Clinton. But even when the thousands of constituent calls, e-mails, and letters flooding Senator Santorum's offices were running 70 percent in support of Clinton, as they did after the House impeachment vote, the fresh-faced lawyer from Pittsburgh remained adamant that the president must stand trial. "You can certainly argue the fact that someone who breaks the law in ... not telling the truth under oath and someone who obstructs justice does in fact threaten the Republic," Santorum said last weekend. Conventional wisdom holds that GOP senators such as Santorum, who come from states that solidly backed Clinton in 1996 and now themselves face uncertain reelection prospects in 2000, will tread lightly on the impeachment issue. Not so, say some seasoned Congress watchers. They maintain that, for a variety of reasons, even politically vulnerable senators like Santorum - who narrowly won his first Senate race in 1994 - are likely to vote their minds. "Anybody who thinks that Rick Santorum or John Ashcroft [(R) of Missouri] or Rod Grams [(R) of Minnesota] are going to vote against impeachment for purely political reasons hasn't met them," says Charles Cook, editor of the Washington-based Cook Political Report. "That's insanity." Strongly held beliefs, rather than political expediency, will hold sway, Mr. Cook predicts. "On something like this, I think people pretty much reach down into their souls." But at what cost will senators vote their conscience against mainstream public opinion? On this, the experts disagree. Arithmetic for 2000 Republicans will be defending 19 Senate seats next year, 13 of them in states carried by Clinton - including several in the Northeast and upper Midwest, where public opposition to impeachment remains strong. The GOP senator seen as facing the stiffest challenge is Mr. Ashcroft, who is being cast by opponents as a hard-liner on impeaching Clinton. Also considered potentially vulnerable are Sens. William Roth Jr. of Delaware and James Jeffords of Vermont, as well as freshmen Santorum and Sen. Mike DeWine of Ohio, both elected in the 1994 Republican "revolution." Democrats, in contrast, will have only 14 seats up for election, and most are viewed as relatively safe, with the exception of Virginia Sen. Charles Robb. Whether the election will alter the current 55-to-45 ratio in the Republican-controlled Senate will depend greatly on who enters the races. Meanwhile, the political fallout from the impeachment will be minimal, some analysts maintain. They contend that Senate seats today are generally safer than they were in past decades. Whereas in the 1970s the incumbent reelection rate was about 60 percent, today it is more than 80 percent. The high expense of campaigns - as well as a trend toward tougher, more offensive strategies by Senate incumbents - makes unseating them increasingly difficult, says Cook. Political impact on senators Moreover, senators, compared with House members, are usually better known by constituents and are therefore less likely to be judged on a single vote. "This isn't going to be a vote that will blow them away," says William Frenzel, a former GOP congressman from Minnesota who is currently a scholar at the Brookings Institution here. Finally, the 2000 election remains far off. "Even under the worst-case scenario no one is predicting this trial will go beyond June, leaving nearly a year and a half before the 2000 election," says Curtis Gans, director of the Committee for the Study of the American Electorate in Washington. "Therefore [the impeachment vote] is likely to have a marginal effect." Others counter, however, that a vote to remove the president is so important it will stay in the voters' minds, and may hurt senators more than their colleagues in the House. Breaking ranks Some senators up for election - ideological moderates such as Olympia Snowe (R) of Maine, or established mavericks such as Roth, for example - may break party ranks on the impeachment vote. But experts again underscore that their decisions will reflect political temperament more than political pressure. "John Chafee [(R) of Rhode Island], Jim Jeffords, and Olympia Snowe are all moderates to begin with," says Doug Bailey, executive publisher of the Hotline political newsletter. "While they might be inclined to seek censure over conviction or [seek to] end the trial early, they are ... likely to do so by virtue of who they are," he said. Open to censure Moderate Republicans such as Senators Snowe and Chafee, for example, have expressed openness to considering censure, an option supported by many Democrats. Some have also voiced reservations on whether witnesses are necessary, again sharing a concern of Democrats. And early on, Senator Jeffords questioned whether a trial was needed at all. A key question remains whether six of the 55 Republican senators would join with the 45 Democrats to create a majority needed to pass important motions - not only on witnesses but on whether to dismiss the case. On this point, experts say that while it is doubtful so many GOP senators would side with Democrats on procedural votes, they may do so on the actual votes on articles of impeachment. "I don't think there will be sufficient votes to short-stop the process," says Mr. Frenzel, although he added that several Republicans could vote "not guilty."