Republican and Democratic presidential candidates are abandoning the middle ground for positions on Iraq and immigration that cater to their parties' fringes _ the latest example of polarization trumping moderation in Washington.

Democrats are lurching to the left on Iraq. Republicans are moving right on immigration. Neither shift is a surprise; the two-party system encourages presidential candidates to appeal to each side's most fervent voters during the nomination fights.

But two things make this cycle special:

- The early start to the 2008 campaign means that White House hopefuls will be listening longer than usual to their parties' ''base'' voters before having to worry about what the rest of the electorate thinks.

- National political leaders are more polarized than they've been in decades.

''It surely doesn't bode well for reconciliation or trying to forge a consensus on these two very important topics,'' said David L. Epstein, an expert on political polarization at Columbia University. ''The worry is that things are just going to become more polarized as times goes on and it's going to be harder and harder for whoever wins to govern.''