WASHINGTON » With four weeks until congressional elections that will shape the remainder of his term, President Barack Obama is increasingly focused on generating enthusiasm within the base that helped put him in the White House two years ago, from college students to blacks.

But Obama has focused much of his prodding — and not a small amount of personal pique — on the liberals most deflated by the first two years of his presidency. Assuming that many independents are out of reach, White House strategists are counting on Obama to energize, cajole, wheedle and even shame the left into matching the Tea Party momentum that has propelled Republicans this year.

As he holds rallies aimed at college students and minority groups, sends e-mail to his old list of campaign supporters and prepares to host a town hall-style meeting on MTV, the president essentially is appealing to his liberal base to put aside its disappointment in him. Without offering regrets for policy choices that have angered liberals, Obama argues that the Republican alternative is far worse.

"You can’t sit it out," he told a conference call of college student journalists last week. "You can’t suddenly just check in once every 10 years or so, on an exciting presidential election, and then not pay attention during big midterm elections where we’ve got a real big choice between Democrats and Republicans."

He added that "the energy that you were able to bring to our politics in 2008, that’s needed not less now, it’s needed more now."

At times, though, the message has come across as scolding and testy, in the view of some Democrats. Obama told Rolling Stone magazine that Democrats "need to buck up" because it would be "inexcusable" for them to stay home. Vice President Joe Biden told a fundraiser recently that the base should "stop whining and get out there and look at the alternatives."

The White House may be making progress closing the so-called enthusiasm gap with Republicans, according to Democratic strategists who point to improving poll numbers and fundraising. But the fact that Obama needs to make such a concerted effort highlights the depth of disaffection among liberals over what they see as his failure to aggressively push for the change he promised.

"It’s great that President Obama is showing a fighting spirit in the weeks before an election, but what his former voters need to see is that same fighting spirit when he’s governing," said Adam Green, a co-founder of the Progressive Change Campaign Committee, a group started last year to advocate for liberal goals and candidates.

David Axelrod, the president’s senior adviser, said the appeal to the base stemmed entirely from political calculations.

"It’s not frustration at all," he said. "It’s fundamental. Almost the entire Republican margin is based on the enthusiasm gap, and if Democrats come out in the same turnout as Republicans, it’s going to be a much different election."

He added: "There are millions of voters who came out in 2008 who were first-time voters who came out because of the president and who aren’t continuing midterm voters. Our challenge is to make them understand this is a consequential election and we need them to participate."

The focus on the left underscores the White House conclusion that it will be harder to convince independents to turn out for Democrats this year.

"The notion of independents is they’re independent and they tend to vote against the majority party," Axelrod said. "I think that is true here."

Recent polls show that Republicans hold an edge among voters likely to turn out on Election Day, while Democrats pull ahead if all registered voters are counted.

But the White House strategy has generated qualms among some Democratic moderates.

Third Way, a Democratic centrist organization, produced a study showing that liberals are the smallest share of the electorate and not enough to keep Congress in Democratic hands. Citing Gallup polling data, the study said self-described conservatives made up 42 percent of the electorate, compared with moderates who make up 35 percent and liberals who make up 20 percent, a shift of several points to the right in the past two years.

In 16 of 21 hotly contested states, Democratic candidates who simply match Obama’s overall 2008 performance still will not have enough votes to win, according to the group’s study. Instead, the study said, the candidates must outperform Obama among moderates.

"Even if Democrats close the enthusiasm gap with their base, they still have another enthusiasm gap to close with moderates," said Anne Kim, domestic policy program director for the group. "Democrats don’t have the luxury of leaning on their base to deliver wins because there simply aren’t enough liberals."

Obama and Biden have stepped up their campaign efforts significantly in recent weeks. Obama will appear at fundraisers in New Jersey on Wednesday and Illinois on Thursday before holding the second of four large rallies Sunday, this one near Philadelphia. Aiming at younger voters, he will hold a webcast town hall meeting Tuesday and two days later another town hall on MTV and five of its sister channels. He has other big rallies scheduled in Columbus and Las Vegas.

Richard Trumka, president of the AFL-CIO, said that it had taken a while for Obama to find a coherent message but that he finally seemed to have done so.

"I see the enthusiasm," he told reporters Tuesday. "I’m out there. I’m doing door knocks. They are focused on things. They understand the importance of this election."

Jim Dean, chairman of the liberal group Democracy for America and brother of Howard Dean, the former Democratic National Committee chairman, said activists were willing to put aside any squabbles with the president for now.

"We’re soldiering on," he said. "We’re going to do this one way or the other. We’re going to work to keep the majority. At the end of the day, whatever issues we have with what the White House says, we can have that conversation on Nov. 3."

© 2010 The New York Times Company