Democrats, for most of the past century, were the more internationalist political party. But now, in a historic transformation, they have become isolationists.

Since Vietnam, Democrats have been mostly reflexive in their opposition to using military force overseas, a trait that explains why their poll ratings on national-security issues are lower than Republicans.

Now, they are becoming the isolationist party on economics as well by taking the protectionist view on trade questions. Congressional approval of CAFTA, the trade pact with Central America, underscores how much the Democratic center has shifted.

There are, of course, exceptions. But they get more difficult to find every day as the party retrenches to reflect its core constituents, who want to believe they can avoid the reality of globalization because of the uncomfortable shifts it will require in our economy.

If CAFTA is the measuring stick, the Democratic exceptions amount to about 10 percent of the party’s members of Congress.

When the Senate approved the deal 54-45, Democrats voted against it by a 33-10 margin.

The legislation, which will lower trade barriers between the United States and Costa Rica, the Dominican Republic, El Salvador, Guatemala, Honduras and Nicaragua got only 15 of 202 Democratic votes when it passed the House of Representatives, 217-215.

Republicans do not monolithically support free trade, but their 43-12 and 202-27 votes in the Senate and House, respectively, in support of CAFTA show the GOP sentiment on trade.

The irony is that for the first 100 or so years of their history, the Republicans were the isolationist party, especially on economic matters.

For the most part, until the past few years, both parties have been committed to free trade.

Little more than a decade ago, when Congress passed the North American Free Trade Agreement, almost half, 102, of the Democratic House members voted for it, as did a large majority of Republicans.

Democratic support for free trade has fallen sharply since. In 2000, when President Clinton, a Democrat, sought approval to normalize trade with China, he won only 73 Democratic House votes, with the majority for passage coming from Republicans. President Bush got only 25 Democrats to give him the ability to negotiate “fast track” deals.

Clearly, Bush’s lack of support on CAFTA has something to do with Democrats not wanting to give him a political victory. But it is much more fundamental than raw partisanship.

Because Democrats are the minority party for the first time since 1932, their hierarchy is increasingly dependent on constituent groups – especially organized labor – who see globalization as a minus, not a plus. They often oppose globalization’s premise that lower trade barriers create jobs overall.

The opponents count the jobs that have been lost to foreign competition, but fail to acknowledge the larger number created in America through the agreements that open up sales of U.S. products overseas.

One big reason for the Democratic opposition is that few of those new jobs are unionized.

And this protectionist wave within the Democratic Party shows little sign of receding.

Most worrisome is that the party’s congressional leadership and the top 2008 Democratic presidential contenders – Hillary Clinton, Joe Biden, Evan Bayh and John Kerry – voted against CAFTA.

If the eventual Democratic nominee makes trade a major issue in 2008, we could see history repeat itself, with a twist.

The Republicans were the protectionist party from their founding in the 1850s. They tried to make protectionist tariffs law after World War I, but Democratic President Woodrow Wilson vetoed them.

Then, in the 1928 presidential campaign, Republican Herbert Hoover was elected on a platform calling for high tariffs, and a GOP Congress then enacted the Smoot-Hawley bill, which is widely blamed for making the Great Depression the most horrific economic event in U.S. history.

The political result of the Great Depression was the rise of Franklin Roosevelt and his Democratic ruling political coalition that dominated American politics through 1968.

CAFTA will be good for the United States, reducing illegal immigration here, propping up emerging democracies in Central America and leading to increased sale of U.S. goods overseas.

But even if Democratic leaders don’t think free trade’s overall value to the American people takes precedence over their union base, they should consider protectionism’s risks to their precarious political situation.

Being the protectionist party will ensure they remain the minority party.