CHICAGO  A cluster of cramped cubicles, tucked away in a rear corner of Senator Barack Obama’s campaign headquarters here, serves as the heart of a fund-raising machine that has reshaped the calculus of the 2008 election.

Mr. Obama’s finance director, Julianna Smoot, who has helped him raise more than $150 million so far, does not even have her own office. A Ping-Pong table is the gathering spot for Friday lunches for her team.

The setting, which has the feel of an Internet start-up, is emblematic of how Mr. Obama, of Illinois, has been able to raise so much money. On Wednesday, the Obama campaign will report to the Federal Election Commission that it collected $36 million in January  $4 million more than campaign officials had previously estimated  an unprecedented feat for a single month in American politics that was powered overwhelmingly by small online donations. That dwarfed the $13.5 million in January that Senator Hillary Rodham Clinton of New York is expected to report Wednesday and the $12 million Senator John McCain’s campaign said he brought in for the month.

Mr. Obama’s startling success, however, has also now put him on the spot, tempting him to back away from indications he gave last year that he would agree to accept public financing in the general election if the Republican nominee did the same. The hesitation has given Mr. McCain, the presumptive Republican nominee whose advisers concede he would most likely fall far short of Mr. Obama’s fund-raising for the general election, fodder for a series of attacks.