After hitting fundraising milestone, Bernie Sanders is ready to seriously contend for the nomination

Mr. Sanders reached a turning point on Wednesday night, when his campaign said that it had raised about $26 million since July — more than Mr. Obama took in for the comparable period in 2007 — and that it had saved enough since the spring to have about $26.5 million in cash.

Mr. Sanders was initially dismissed by political insiders as a fringe candidate running only to push Hillary Rodham Clinton to the left. But he has now demonstrated that he has the resources and the supporters, whom he has only begun to tap financially, to compete for the Democratic presidential nomination.

The great unknown is whether Mr. Sanders can turn his ideologically focused, frugally run movement into a national campaign: Can he expand beyond a fiercely liberal base and attract the broader cross-section of Democrats required to beat Mrs. Clinton, and possibly Vice President Joseph R. Biden Jr.? While Mr. Sanders has 92 paid workers and 24 offices across the early battlegrounds of Iowa and New Hampshire, his operation is still at heart that of a man with a message, lacking the network of political allies across the country and in Congress who can help him build get-out-the-vote organizations that win elections.