New presidents habitually — and often, mistakenly — try to do the exact opposite of what their predecessor did. But after George Bush’s record as “the Great Polarizer,” President-elect Obama definitely should seek to be “the Great Unifier.”

[IMGCAP(1)]Just by getting elected as our first African-American president, he’s gone a long way toward healing the country’s ugliest historical division.

Obama’s victory speech in Chicago’s Grant Park Tuesday night was the capstone of a campaign built around the theme of ending partisan and ideological rifts in the nation, too.

But while he may believe, as he said, that “in this country, we rise or fall as one nation, as one people,” other people — starting with members of his own party — will have to agree to “resist the temptation to fall back on the same partisanship and pettiness and immaturity that has poisoned our politics for so long.”

Obama promised he would listen to those who disagree with him — Bush often didn’t — and reached out to “those Americans whose support I have yet to earn. … I hear your voices. I need your help. And I will be your president, too.”

Of course, they all say that. Bush promised to be a “uniter, not a divider,” and proved to be the exact opposite, abandoning “compassionate conservatism” within months of taking office.

But Obama has been calling for national unity for four years now — beginning with his stirring Democratic National Convention speech in 2004 — and presumably means to deliver on the promise.

He also has to know that the problems Bush has handed off to him — financial crisis, crushing debt and multiple foreign challenges — cannot be solved if he doesn’t at least seek input from his adversaries.

It’s a boon to him that he’s approaching the long, steep climbs ahead with a Reaganesque optimism based on faith in America’s capacity to renew itself. The “yes, we can” spirit is bedrock American, a unifier all unto itself.

And it was a gift — not unexpected, given his “country first” life’s record — that Sen. John McCain (R-Ariz.) gave a concession speech urging his supporters to join him in offering Obama “earnest effort to find ways to come together, to find the necessary compromises, to bridge our differences.”

But good intentions are one thing. Delivery is another. Obama particularly warned the Democratic Party that, while it won a “great victory” Tuesday, it has only been given “the chance to make the change we seek,” and “that can’t happen if we go back to the way things were.”

In fact, the Democratic victory was not that great. Maybe there was a “wave,” but a gain of fewer than 20 seats in the House and perhaps only five in the Senate is not a “tsunami,” especially given the fact that 71 percent of voters told exit pollsters that they disapprove of Bush.

An even greater number, 73 percent, said they disapprove of the job Congress is doing. And though Sen. Charles Schumer (N.Y.), the Democrats’ Senate campaign chief, last week foresaw a generation-long “tectonic shift” in the works, there’s been no fundamental realignment.

Democrats have gained seats in two successive elections, but they should remember some history. Republicans were given up for dead in 1964, but they picked up 47 House seats in 1966 and regained the presidency in 1968, according to Vital Statistics of Congress 2008.

Richard Nixon carried 49 states in 1972, but his party lost 49 seats after his resignation in 1974 and Democrats regained the presidency in 1976, only to lose it in 1980, when Republicans gained 34 seats.

Ronald Reagan did effect something of a Republican realignment from 1980 to 1992 that was reinforced by the GOP takeover of Congress in 1994, which lasted 12 years. But that’s gone now, too.

The historical lesson of Presidents Lyndon Johnson, Nixon, Jimmy Carter, Bill Clinton and Rep. Newt Gingrich (R-Ga.) is: Those who mess up — usually by overreaching — lose. As Obama said, Democrats have an opportunity now. They have not been anointed.

It’s a good opportunity. Obama was carried to victory on the strength of emerging demographic groups — young people, Latinos and African-Americans. The future is on his side.

But whites older than 30 still represent 63 percent of the electorate and they voted 57 percent for McCain. Obama won because that margin was somewhat smaller than Bush’s in 2004.

The fundamental fact of politics — as this year’s exit polls demonstrated once again — is that 44 percent of the electorate regards itself as moderate, 34 percent as conservative and only 22 percent as liberal.

Moderates supported Obama by 61 percent to 38 percent this year. They want the parties to work together to solve America’s problems. Obama needs help from both Democrats and Republicans.