WASHINGTON — President Barack Obama collected $45 million for his re-election bid in February, accelerating his fundraising pace as his campaign frets over an oncoming spending blitz by Republican-leaning outside groups.

Obama’s monthly haul was nearly twice as much as the $23 million per month average he raised during the final three months of 2011 and more than the $29.1 million he raised in January. Yet the largesse still fell short of the $56 million he raised in February 2008, when he was seeking the Democratic nomination against Hillary Rodham Clinton.

With Republicans locked in an extended primary, Obama’s team has tried to build a large 50-state operation that will help it register new voters, bring back past supporters and boost turnout. Obama’s campaign had about $75 million in the bank through the end of January; totals for February were not immediately available.

Campaign officials have implored supporters to donate money and get involved, pointing to Republican-leaning super PACs expected to raise hundreds of millions of dollars to defeat the president. Obama’s campaign said earlier this year that it would bless big-money super PACs supporting Democrats as a way of countering the Republican effort.

In an email to donors last week, campaign manager Jim Messina cited a poll showing Obama trailing Republican Mitt Romney and asked them to get involved.

“We’re looking at a race that will be tighter than you think. And the other side has groups ready to spend hundreds of millions of dollars to tear down President Obama,” Messina said.

Obama has consistently outraised his Republican opponents. Romney, who has led the Republican field in fundraising, brought in $6.5 million in January, about a quarter of what Obama raised that month.

But Obama’s totals in February lagged behind what candidate Obama raised four years ago, before he had the benefit of the entire Democratic Party apparatus behind him.

Kirsten Kukowski, a Republican National Committee spokeswoman, said Obama was “having a hard time convincing voters he deserves another term” following three years of “record debt, high unemployment, and soaring gas prices and healthcare costs.”

Obama’s campaign said nearly 350,000 people contributed in February and the average donation was about $59 for the entire election cycle. Nearly 98 percent of the donations were $250 or less. The money was spread among Obama’s campaign, the Democratic Party and two campaign funds.

Obama has boosted fundraising efforts in recent weeks, holding events last month in Miami, Los Angeles, San Francisco and Seattle. Last week, Obama raised money in Chicago and Atlanta.

The fundraising reports were being filed ahead of a Tuesday campaign finance deadline for presidential campaigns.