By Alina Selyukh and Alexander Cohen
WASHINGTON (Reuters) - Restore Our Future, an outside "Super PAC" backing Republican presidential hopeful Mitt Romney, raised $3.9 million in April, less than half of its haul in March but still far ahead of its Democratic counterpart working to re-elect President Barack Obama.
The pro-Romney "super" political action committee Restore Our Future had $8.2 million in cash on hand at the end of the latest reporting period, filings with the Federal Election Commission showed on Sunday.
The pro-Obama group, Priorities USA Action, raised $1.6 million last month and had $4.7 million cash on hand - most of which has since been spent on an ad blitz against Romney's private equity past, disclosure forms showed.
In March, Restore Our Future had $6.5 million in cash and Priorities USA, $5 million.
Both Democrats and Republicans are racing to raise as much money as possible for advertising and get-out-the-vote efforts ahead of the November 6 election. Totals are projected to reach as much as $1 billion for each.
The pro-Romney Super PAC has raised more than its Democratic rival every month since they launched, helping the Republican candidate gain ground on the well-oiled fundraising machine of the president's campaign.
Another highly influential Super PAC helping Romney is American Crossroads, which raised $1.8 million and reported $25.5 million in cash to spend. The group has a sister non-profit Crossroads GPS that is not required to disclose its results or donors.
Super PACs, spawned by a 2010 U.S. Supreme Court decision, have already spent more than $100 million trying to influence congressional and presidential races this campaign cycle.
Many Democrats opposed the court decision and have remained hesitant to donate to the groups that have largely taken over the dirty work of negative advertising.
Restore Our Future, which counts more than a dozen donors who have given $1 million or more, last month scored another seven-figure donation from Fort Worth, Texas, hedge fund manager John Kleinheinz.
Harold Hamm, chief executive of oil production firm Continental Resources Inc., who recently became Romney's top energy adviser, contributed $985,000.
Stephen Zide, a managing director at Bain Capital LLC, a private equity firm co-founded by Romney, gave $250,000.
Houston homebuilder Bob Perry, who is one of the Super PAC's biggest donors and a known Republican financier, last month gave Restore Our Future another $750,000 but the filing shows that the donation was returned several days later.
Perry's fellow top Republican donor, Texas billionaire banker Harold Simmons give another $1 million to American Crossroads in April, for a total of $11 million in personal contributions to the group and another $2 million from his industrial manufacturing firm Contran Corp.
Priorities USA for its part garnered $1 million from the National Air Traffic Controllers Association labor union.
The group's top individual donor was Masimo Corp founder and Chief Executive Joseph Kiani who gave $100,000. Kiani is one of Obama's campaign fundraisers, or "bundlers," and his company has lobbied the administration on a medical device excise tax opposed by the industry that is included in the 2010 healthcare reform law.
Last week, Priorities spent $4 million on an ad airing in key swing states focused on a Kansas City steel mill that went bankrupt under Romney's private equity firm Bain.
Romney emerged as the presumed Republican nominee in April as he started fundraising alongside the Republican Party.
Fred Zeidman, one of Romney's top fundraisers, said the campaign has been focusing heavily on raising funds for the joint fund, offering big donors a new venue to give large sums to Romney's effort aside from the Super PAC.
Super PACs can take unlimited funds, campaigns can accept $2,500 once during the primary battle and again during the general election, and joint funds can accept up to $70,800 in addition to those campaign contributions.
Last month, Obama's campaign and Democratic allies raised $43.6 million and said there was $115 million in the bank. Romney and the Republican party said they raised $40.1 million and had $61.4 million left in cash on hand.
Two of Romney's rivals - Ron Paul and Newt Gingrich - ended their challenging bids in May, after Paul raised $2 million and ending April with $2.5 million left on hand and no debt.
Gingrich dropped out on May 2 after closing out April with $806,951 in cash on hand and $4.8 million in debt. His campaign raised $638,831 last month.
Santorum, who ended his campaign on April 10, raised $1.2 million that month, ending it with $1 million left in cash on hand and $2.3 million in debt.
American Crossroads, which is working alongside its non-profit sister group Crossroads GPS, plans to spend up to $300 million this campaign season, and two-thirds of it on defeating Obama.
The Crossroads groups were conceived in part by Karl Rove, chief strategist of Republican George W. Bush's two presidential election wins. Crossroads GPS, the non-profit, is not legally required to report its fundraising or donors to the FEC.
(Additional reporting by Patrick Temple-West; Editing by Eric Walsh)