Democratic Candidates Moving Left

"When we began this campaign the consensus on the Democratic side was that only incremental change was possible," Borosage said. "What is striking is how quickly we have made progress. So John Edwards put out a comprehensive health care plan, abandoning the question of incrementalism, Sen. Obama soon followed. Sen. Clinton has promised that she will do so over the next few weeks."

Some party strategists note that the Democratic candidates are not embracing the extreme left. No major Democratic candidate is endorsing gay marriage, single-payer health care, or an immediate and total withdrawal from Iraq -- positions that have sizeable, if not overwhelming, support among Democratic primary voters.

The Democratic Race and the Direction of the Country

Still, the Democratic candidates appear to be taking the attitude that winning the primary will virtually ensure them of winning the general election, since the country appears to be so soured on the Republican party.

"At the end of the day, it is a Democratic race. People are not happy with the direction of the country," said Chris Lehane, a Democratic consultant and a former top aide to Al Gore in 2000. "The match-ups will look very different when these other candidates come into focus."

Berry said the move to the left is in a broad sense a return to the Democratic party's roots, since the various interest groups that hold sway over the party have traditionally wanted a more liberal take on the major issues of the day.

But the candidates must balance the need to tap into activists' energy with the necessity of finding common ground with political independents.

"They'll get the Democratic base to vote for whoever the Democrats nominate, but they have to win over the independents," Berry said. "They're going to have to resist some of this leftward tug."

ABC News' Nik Bonovich contributed to this report.