WASHINGTON  With four weeks until Congressional elections that will shape the remainder of his term, President 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 African-Americans.

But Mr. Obama has aimed much of his prodding  and not a small amount of personal pique  at 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 Mr. 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, Mr. 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.”