Voting records and campaign pronouncements are the coin of the realm for pundits trying to predict how a presidential contender will lead once he moves into the Oval Office.

But presidential scholars say congressional votes and campaign promises are rarely an accurate predictor of presidential behavior, in large part because the differences between being a legislator and being an executive are so great.

And those indicators are even less helpful, historians said, in predicting how Sens. Barack Obama, D-Ill., and John McCain, R-Ariz., would handle immigration policy because there is little in their congressional voting records and stump speeches to set the two apart.

“Once you’re president everything changes, there are wholly are different set of pressures and wholly different consistencies,” presidential historian Allan Lichtman of American University said in an interview. “You’re no longer representing Illinois or Arizona, but a national political party and an entire nation.”

Some change is expected because the transition from Congress to the executive is an evolutionary process, like a child growing up, according to David Abshire, president of the Center for the Study of the Presidency.

But while former congressmen-turned-presidents can take on issues of broader interest when their constituency is the entire nation, there are still institutional constraints. Foremost among those are the demands of party constituencies.

“A president is not just a free-wheeler. He’s got to be a party leader,” Abshire says, “He can’t lose his base, but at the same time he can’t just default to the base and let it lead him.”

Lichtman said that history proves congressional votes and reputations don’t translate once you’re president.

“The conventional wisdom is that people who serve in Congress learn to compromise, to cut back,” he says, “But if you look at [Lyndon B. Johnson], who was the ‘Master of the Senate’ and [John F. Kennedy], they were both strong executives.”

“LBJ had a very conservative voting record [in Congress], he was a Southern Democrat. He wasn’t what you’d call progressive on civil rights,” says Thomas Whalen, a Boston University professor and author of “A Higher Purpose: Profiles in Presidential Courage”.

But when he became president, he helped further a social revolution by supporting the Voting Rights Act (PL 88-352), which banned discriminatory voting rules that had disenfranchised African-Americans, Whalen says. “He surprised everybody, particularly in the South.”

A prior congressional career might, however, inform a president’s conception of executive power. A career as a legislator could help a future president develop greater skill in working with Congress and a greater willingness to reach across the aisle, experts say.

“Coming from the Hill helps with the congressional interface,” Abshire said.

Presidents, he said, “build effective power by reaching out and building coalitions.”

He contrasts that with what he says is the style of President Bush — a former governor who never served in a legislature: you build executive power by pushing forward without yielding to the other side.

Kennedy, who spent 14 relatively undistinguished years in Congress, exemplifies how legislative experience can instill a willingness to work with the other party and the legislative branch.

“JFK, in his very brief career, was very good with Congress,” Abshire says. “He did have a bipartisan cabinet and he did operate with bipartisan skill on Capitol Hill and it served him very well there.”

So if we can’t assume that the voting patterns McCain and Obama have cast will necessarily follow them into the Oval Office, what can history tell us about their presidential styles?

McCain “has been shaped by many years in the Senate,” says Christopher C. Hull, a Georgetown University professor and author of “Grassroots Rules: How the Iowa Caucus Helps Elect American Presidents”.

But McCain isn’t a typical senator.

“He’s not a back-slapping, bridge-builder,” Hull said. “He’s not very good with personal relationships. But he is a flame-thrower.”

He says that could mean that a McCain presidency could be characterized by bipartisanship.

“You can project that if [McCain] were elected and that we’re going to have a Democratic [congressional] majority, he will be able to work across those party lines,” Abshire says.

Obama, on the other hand, might benefit from a brief stint in the Senate, Hull said.

“LBJ — a very powerful U.S. senator very good at cutting deals [in Congress] — was a terrible president because the presidency is about leadership,” offering a different take on Johnson than did Abshire.

Experts also say that McCain’s staunch support of the doomed comprehensive immigration bill (S 1348) last summer and Obama’s wide margins with Latino voters in pre-election polls will have less to do with how they would tackle immigration policy as president than party demands and partisan congressional dynamics.

Lichtman says that immigration is the most difficult issue for candidates in the 2008 presidential race because it has moved to the forefront of the nation’s consciousness, with the public still very divided over what to do about it.

“[Immigration] is such an emotional issue and there are no clear answers,” he said. “On this difficult and dangerous issue, they are looking at the pressure from their parties.”

Frank Sharry, executive director of America’s Voice, a group that promotes candidates favoring a comprehensive immigration overhaul, said, “McCain has been, indisputably, a visible champion . . . and put his career on the line. He has the edge as far as legislative experience.”

But Lichtman says that instead of remembering his support for the overhaul, his Republican affiliation “will be perceived as hostile to immigration or Hispanics” by Latino voters.

He predicts that Obama will win the Latino vote 2 to 1 over McCain.

Obama knows that immigration “is not a win-win for him. He’s going to get the reform vote anyway, and because it’s an issue where you can alienate a lot of potential voters, moderates and cross-overs,” keeping the issue a secondary plank of his platform is a strategic decision.

So while Obama won’t benefit from clarifying his immigration stance and McCain risks alienating the border-security first wing of his Republican base by boasting of his comprehensive overhaul credentials to Hispanics, experts say it’s doubtful we’ll get a clear picture during the campaign of how either would handle the issue as president.

Indeed, it is possible that party forces will determine whether either contender would make immigration overhaul a priority after taking office.

Whalen said the status quo would be more politically safe for both potential presidents.

“They’ll expect the states will enact anti-immigration laws and they’ll be kicked up to the Supreme Court to be decided,” he says.

This hands-off tactic is how President Richard M. Nixon was able to hold onto his Southern support while implementing desegregation policies mandated by the Supreme Court.

In effect, Whalen said, Nixon was able to say, ‘I’m personally against busing, but this is what the Court has said I must do.’

Obama and McCain could follow the same tactic on immigration, with the party factions standing in for the Supreme Court as fall guys, Whalen says.

One activist who has disagreed with McCain’s approach in the past is not as sanguine about his inability to drive the issue as some of the scholars are. Another who has praised him fears, in contradiction of the scholars’ assertions, that his campaign rhetoric will box him in.

“McCain is much more emotionally committed to pushing amnesty. Amnesty for him is not businesses anymore, it’s personal,” said Mark Krikorian, executive director of the Center for Immigration Studies. “He wants to get back at Republicans for embarrassing him” when comprehensive overhaul failed last summer.

In Krikorian’s estimation, Obama and McCain believe most of the same things about immigration, but would probably behave differently.

“It’s not all that important to [Obama]. He’s more likely than McCain to give up,” Krikorian said.

From the other end of the spectrum, Sharry said that while McCain has an immigrant-friendly reputation, his campaign position that border security has to be achieved before changing the immigration system would limit what he could achieve on the issue as president. The border security caveats have “contorted his position. He has painted himself against a wall.”

Sharry said McCain’s proposal for border state governors to certify whether the border is secure would be nearly impossible in practice.

“This phony certification process will come under intense pressure,” Sharry says. “He’s handed a veto to governors who will not play a weak-kneed game. This precondition pretty much makes [comprehensive immigration overhaul] impossible,” Sharry said.

All that assumes, of course, that Obama and McCain’s current pronouncements aren’t subsumed by the demands of party or changed circumstances in 2009 and beyond — something the presidential scholars clarified it is not safe to assume.

Another point operating on the side of uncertainty: the small universe of examples upon which scholars have to draw to reach their conclusions.

Only two sitting senators have been elected president. Americans have not even sent a former senator to the White House since 1972.

“This is the first time that we’re going to elect a U.S. senator straight from Congress since 1960,” Hull says. “We’re kind of in a brave new world now.”

Caitlin Webber can be reached at cwebber@cq.com.