Washington ? The independence being demonstrated all over the political spectrum these days – by Democratic Sen. Joe Lieberman and Republican Sen. Lincoln Chafee, both in tough re-election battles, and by Republican Sens. John McCain and John Warner – has its roots in American history. When they ran for the presidential nomination, Ronald Reagan and John Kennedy defied the preferences of their parties’ power brokers. And earlier, Teddy Roosevelt and Abe Lincoln did the same thing.

The American people often have been equally ready to discard conventional wisdom by rewarding independence and those candidates who, in the words of the Declaration of Independence, show “a decent respect to the opinions of mankind.” In my view, and that of many others, had Colin Powell chosen to run for president in 2000, in either party or in none, he could have been elected – his character and reputation overcoming any partisan ties and any lingering racial prejudice.

Instead, we had George W. Bush, a president who has governed on party-line votes, and who, after taking the country to war in Iraq, repeatedly has used the war on terror as a partisan weapon against Democrats.

Powell will not run in 2008, but if this year’s election strengthens the hands of the independent members of both parties – those who are prepared to defy the dictates of their interest groups and clamorous extremes – the next presidential race may be very different from recent cycles.

Who could provide such a contest? On the Republican side, either of the two men who sit atop the early polls, McCain or former New York Mayor Rudy Giuliani. Giuliani has flourished in Democratic New York by defying the rigid social-issues doctrine of his party. McCain has challenged everything from the first of President Bush’s deep tax cuts to Bush’s recent demand for a free hand in questioning and trying alleged terrorists.

I also would guess that Massachusetts Gov. Mitt Romney would be such a candidate. Like Giuliani, he has governed in Democratic territory, and he managed with an overwhelmingly Democratic Legislature to pass a major step toward universal health care coverage.

On the Democratic side, Sen. Hillary Clinton would, if she runs, provide a crucial test case for the independence movement. Though tied to some of the party’s most important constituencies, she has, like her husband before her, carved her own path in policymaking. Clinton is not alone among possible Democratic aspirants in showing her independence. Others, such as Iowa Gov. Tom Vilsack and former Virginia Gov. Mark Warner, have demonstrated their ability to conciliate differences without abandoning principles. And the field of potential candidates who are moderate is a large one.

A race involving any of these people would provide more substantive debate than cheap shots and demagoguery.

And looking beyond 2008, think about a future contest involving Democratic Sen. Barack Obama of Illinois and Republican Sen. Lindsey Graham of South Carolina – or any of many other pairs of younger talents, with similarly inclusive and expansive views of the national interest. What a boon for the country.

But is this rosy scenario likely? Look at the powerful forces working against it. Congress is rigged to promote partisanship and extremism. Most congressional districts are drawn to favor one party or the other, and contests take place only in primaries, where low turnouts favor candidates who appeal to the motivated extremes. The flow of special-interest money into congressional races adds to this tilt, and blogs are pummeling anyone who deviates from their definition of ideological purity.

The sequencing of presidential primaries, it is said, has the same effect on the race for the White House. Democratic aspirants have to satisfy the lefties to win Iowa, just as Republicans must placate the religious right to have a chance in South Carolina.

All that may be true. But still the forces of the independent center are gaining. The public disgust with the breakdown of Congress as a functioning institution has liberated more House and Senate candidates to challenge the status quo. They may be the same people, but they’re not behaving the same way.

And the political environment is changing. More and more traditional conservatives are complaining that the Bush administration is wrecking their heritage, with its reckless military, foreign and fiscal policies and its disregard for the law. I hear this regularly and have reported it. David Brooks has been making that same point in his influential New York Times columns.

The tide is turning against dogmatism – and toward political independence.