President Obama is struggling to draw in big-dollar donations, with half as many people writing large checks to his campaign than at this point four years ago.

Obama is outpacing his Republican rivals in fundraising overall, and his advisers have concentrated on amassing small-dollar backers, part of a strategy to get more people invested in the reelection effort. At the end of January, 1.4 million people had donated to the Obama campaign, responding to appeals for contributions as small as $2.

But Obama lags behind Republican front-runner Mitt Romney in finding donors willing to give $2,000 or more — a surprising development for a sitting president, and one that could signal more worrisome financial problems heading into the general election. At this point in the last election cycle, Obama had received such large donations from more than 23,000 supporters, more than double the 11,000 who have given him that much this time. President George W. Bush had more than four times that number of big donations at this point in his reelection.

Democrats see a variety of possible explanations for such a dramatic drop in big-dollar contributions. The ailing economy has dampened fundraising overall. Some wealthy liberals and Wall Street executives alike have grown disaffected with the president over time. And the extended Republican primary has shined a spotlight on a field of potential rivals that many Democrats believe Obama will easily beat.

“Some people think these Republicans are easy marks, and they aren’t taking it as seriously as they need to yet,” said Judy Wise, one of Obama’s “bundlers,” the campaign term for people who host events and gather checks from other donors.

Whatever the reason, Obama appears to be redoubling his efforts to extract bigger contributions from his support base. He has stepped up his fundraising events in recent weeks, taking swings through several different regions for more than 40 events in 2012. On Friday, the president did five events in two states with an expected haul of at least $5.5 million.

The campaign typically holds three or more events with different donation levels in one evening, part of a strategy to make Obama available to different types of people. The biggest event Friday featured singer Cee Lo Green at Tyler Perry Studios in Atlanta. Tickets were $500 with a $2,500 donation for VIP access and a $10,000 donation for a photo with the president.

Federal laws cap donations to the candidates at $2,500 for the period before the convention and $2,500 after that. Obama has been able to raise much bigger amounts by collecting donations for the Democratic National Committee, which can total $61,600 per person over two years.

Republicans and Democrats alike thought Obama would have a big financial advantage over Republicans this fall given his record-breaking 2008 fundraising and his status as the sitting president. But the trend of slackening big-donor support is the latest in a series of indications that the 2012 money battle is going to be much tighter than once imagined.

Some major Obama bundlers said in interviews that they are having trouble drumming up the same level of excitement that surrounded his bid four years ago. At the start of that campaign, Obama was an underdog running against Hillary Rodham Clinton in the Democratic primary and there was a potent sense of urgency to fundraising appeals.

Obama has raised much more money that his Republican competitors by not only courting small contributors but also asking for bigger checks. Obama and the DNC have raised a combined $255 million since the president launched his campaign, compared with Romney’s $63.1 million total.

Obama campaign officials describe his donation base as an asset; smaller donors means a more grass-roots vibe, they said.

“Our fundraising numbers reflect the grass-roots support the president has received from across the country,” campaign spokeswoman Katie Hogan said. “We are building the largest grass-roots campaign in history, and these donations will go towards investing a ground-up, national organization.”

Still, recent moves by the Obama campaign have suggested that it may be eventually pinched for cash. Obama campaign manager Jim Messina and White House adviser David Plouffe told top congressional Democrats that Obama and the DNC don’t have any money to spare for House and Senate races.

Obama also recently changed course and asked top fundraisers to support a super PAC acting on his behalf. He has criticized the million-dollar contributions that fuel super PACs, a new type of political group, but decided that he couldn’t afford to take the beating from PACs on the right without responding in kind. So far, the main super PAC behind Obama — Priorities USA — has posted anemic fundraising, in contrast to the millions of dollars conservative groups have already begun spending against him.

At the same time, the Obama campaign has been burning through money at a furious pace. In January, for example, the Obama campaign spent $17.7 million while raising only $11.9 million. That has left Obama with about half as much money in the bank as Bush had eight years ago ahead of his successful reelection.

At the end of January, President Obama and the DNC had $74 million on hand for the period before the conventions, according to the latest Federal Election Commission reports. Bush and the Republican National Committee had $144 million at the same point in 2004.

Some bundlers have decided to stop supporting Obama entirely, including several in the finance sector, which has been hit with stringent new regulations pushed by Democrats.

“There’s a lot of disaffection and buyer’s remorse among the people I know,” said one 2008 Obama fundraiser, who is no longer working for the president and was interviewed on the condition of anonymity in order to speak freely. “At the end of the day, would they vote for him? Maybe, but they’re certainly going to be less active.”

The erstwhile bundler said he would probably prefer to support Democrats in the Senate, who are in danger of losing their majority. He said he might change his mind and cut a check for Obama if he thought the president really needed the money or a candidate other than Romney were nominated by Republicans.

According to a Washington Post analysis of financial reports filed by the campaigns, more than 11,000 people have donated $2,000 or more to the Obama campaign or the Democratic National Committee since the start of the race. More than 23,000 had given that much at this point in 2008. And more than 49,000 did the same for Bush and the Republican Party by this point in the 2004 cycle.

“There is an issue with Democratic complacency,” said one Democratic strategist not working for the campaign. “Far too many Democrats think that the president can’t lose to Mitt Romney — and he definitely could.”

Jack Oliver, Bush’s national finance vice chairman in 2004, said the president’s big donors should be galvanized by polling and economic data pointing toward a tight general election contest.

“The bundlers should be motivated right now because everybody knows that early money is valuable,” Oliver said. “It allows you to plan.”

Part of the reason that big donors have not rushed back to the campaign could be the president’s own style. Obama wrote in a 2006 book that he doesn’t like asking rich people for money, a contrast to many politicians of his stature who can gleefully work a room of donors.

“He’s not someone who needs and therefore relishes this kind of direct personal contact,” said Charles Lewis, a Chicago area philanthropist and a member of Obama’s national finance committee. “People who attend get more out of it than he does.”