President Obama’s re-election campaign is straining to raise the huge sums it is counting on to run against Mitt Romney, with sharp dropoffs in donations from nearly every major industry forcing it to rely more than ever on small contributions and a relative handful of major donors.

From Wall Street to Hollywood, from doctors and lawyers, the traditional big sources of campaign cash are not delivering for the Obama campaign as they did four years ago. The falloff has left his fund-raising totals running behind where they were at the same point in 2008 — though well ahead of Mr. Romney’s — and has induced growing concern among aides and supporters as they confront the prospect that Republicans and their “super PAC” allies will hold a substantial advantage this fall.

With big checks no longer flowing as quickly into his campaign, Mr. Obama is leaning harder on his grass-roots supporters, whose small contributions make up well over half of the money he raised through the end of March, according to reports filed Friday with the Federal Election Commission. And Mr. Obama is asking far more of those large donors still giving, exploiting his joint fund-raising arrangement with the Democratic National Committee to collect five-figure checks from individuals who have already given the maximum $5,000 contribution to his re-election campaign.