America needs a true “uniter, not a divider” as its next president, but candidates who might fit the bill are rapidly kicking away the opportunity by pandering to their parties’ ideological base.

At the top of the list is Sen. Barack Obama, D-Ill., who has made an end to “partisan bickering and ideological decision-making” the keystone of his campaign, but on issue after issue has been pandering to the left and undermining his chances to be a unifying president.

No one else in the race has directly promised “new politics” as Obama has, but Sens. Hillary Rodham Clinton, D-N.Y., and John McCain, R-Ariz., former New York Mayor Rudy Giuliani (R) and former Massachusetts Gov. Mitt Romney (R) all have records of operating across party lines that they are abandoning to one extent or another.

Of the lot, only McCain is emerging as a true “conviction politician” willing to risk defeat for his beliefs, notably by defying his party’s base to back bipartisan immigration reform and by bucking the country’s distaste for the Iraq War by backing President Bush’s troop “surge.”

Bush promised in 2000 that he would be a “uniter, not a divider” and would “restore civility to political discourse.” But he’s done the exact opposite, appealing primarily to the GOP base with minimal outreach to Democrats.

As a result, if Henry Clay goes down in history as the “Great Compromiser” and Ronald Reagan as the “Great Communicator,” Bush deserves to be tagged the “Great Polarizer” — detested by Democrats and, increasingly, abandoned by independents.

After the 2006 elections, both Bush and newly victorious Democrats said voters sent a message: They were fed up with partisan warfare and wanted bipartisan action on pressing national problems.

It should be obvious that only on a bipartisan basis will major problems get solved, particularly because it requires 60 votes for any controversial bill to pass the Senate.

If the next president wants to fix health care, education, the tax code, energy policy or the nation’s long-term fiscal mess — and, maybe, immigration, if current efforts fail — it can only be done by brokering a set of “grand bargains” between interest groups and the two parties.

Obama, in particular, has advanced the theme — at least rhetorically. In Reno, Nev., at the end of May, for instance, he declared that “America wants a new message. America wants a new spirit. America wants to turn the page on a selfish politics, a small politics, on a politics obsessed with who’s in power and who is not and partisan bickering and ideological decision-making.”

But then, on the floor of the Senate last week, Obama voted for one amendment — backed by the AFL-CIO and sponsored by Sen. Byron Dorgan, D-N.D. — designed to insert a deadly “poison pill” into the bipartisan “grand bargain” on immigration reform. And he sponsored another.

And when Obama unveiled his healthcare reform proposal, he went out of his way to demonize health insurance and pharmaceutical companies that would have to be part of any bipartisan compact to lower costs and expand access.

Clinton, too, voted the AFL-CIO line on immigration, voting to kill the guest-worker program. And, despite her experience losing the 1994 healthcare reform battle to insurance companies, she has begun making them the enemy once again.

On Iraq, both Obama and Clinton have abandoned past opposition to hard withdrawal deadlines and funding cutoffs in an apparent effort not to be outflanked on the left by former Sen. John Edwards, D-N.C., who also is trying to wrap up the populist and labor vote among Democrats.

Democrats are far from alone in pandering and polarizing. Romney, in particular, has abandoned previous stances on abortion, gay rights and immigration to appeal to the GOP base vote.

Giuliani has stuck to his previous pro-choice position on abortion but has abandoned his support for broad immigration reform to appease border-security restrictionists.

In the most recent GOP debate, Giuliani denounced the pending Senate immigration bill as a “typical Washington mess.” In fact, it was a delicate compromise between conservatives and liberals — the kind of “mess” it will take to solve nearly every problem the next president will face.

McCain has done his share of repositioning, too — notably on taxes. McCain voted against Bush’s exorbitant tax cuts in 2001 and 2003 and now wants them extended, a clear cave to the GOP base.

But on Iraq he has been a consistent truth-teller, arguing that the president committed too few troops to win the war. And on immigration, he’s been stalwart in the face of rage in the base stirred up by radio talk-show hosts.

It’s a dictum of American politics that GOP candidates “go right” — and Democrats, “left” — in the primaries and try to reclaim the center in the general election. No doubt, that’s the plan many 2008 candidates hope to execute in this race.

The danger, though, is that by pandering to the base, they will become locked into positions that they can’t retreat from in the general. And that they’ll poison the atmosphere needed for post-election consensus-building.

The 2008 candidates should try to avoid the unhappy pattern of George Bush, but they are in the process of repeating it.