President Obama burned through twice as much money in June as Republican challenger Mitt Romney, who ended the month with more cash on hand and has staked out a dominant position in the presidential money race.

Pushing to define Romney and respond to attacks from conservative groups, Obama spent nearly $58 million in June, far more than any previous candidate has spent at this point in the campaign. Romney’s team, by comparison, spent $27.4 million.

The numbers, in disclosures filed Friday with the Federal Election Commission, underscore how rapidly the fundraising contest has been transformed since Romney was all but assured the Republican nomination in April with little money remaining from a brutal primary fight.

Obama, whose fundraising juggernaut in 2008 became political legend, has had a harder time raising money this time and has been outraised by Romney for two straight months. The presumptive Republican nominee and affiliated party committees had $170 million cash on hand at the end of June, compared with $147 million for Obama and the Democrats.

Presidents George W. Bush and Bill Clinton were outraised by their challengers, but Obama campaign officials say they are particularly worried because of an influx of money from conservative groups that can amass unlimited amounts. Last week, for example, the Obama operation spent twice as much as Romney on advertising but was still outspent by $1.9 million when independent groups were added to the mix, according to data from Kantar Media/CMAG.

The reports filed Friday reflect two distinct fundraising approaches: Obama relies more heavily on smaller contributions and keeps most of his cash under the control of his campaign, while Romney focuses more on high-dollar donors and has most of his money stashed at the Republican National Committee.

Even so, Romney was able to increase the amount he raised from small donors, considered an important measure of grass-roots support. In June, his campaign and the RNC raised 22 percent of their money from donors who gave less than $200 at a time, up from 14 percent a month earlier. That’s still less than half the rate of Obama and the Democratic National Committee, which have raised 47 percent of their money from such donors.

Obama on Friday identified more than 100 new fundraisers in the second quarter who had bundled together $50,000 or more in contributions. The full list includes 638 bundlers who have raised at least $143 million for the campaign and the DNC, records show. Romney has not released a list of his bundlers.

Restore Our Future, a pro-Romney super PAC run by former Romney aides, reported $20.7 million in donations in June, nearly half of which came from $5 million gifts from casino magnate Sheldon Adelson and his wife, Miriam.

The pro-Romney super PAC also received $200,000 from Scotts Miracle-Gro, a publicly traded lawn-and-garden conglomerate based in Marysville, Ohio.

Sean Fieler, a financial analyst at Equinox Partners, gave the group an unusual $50,000 contribution in the form of stock, FEC records show. Other big givers to the super PAC in June included Texas home builder Bob Perry ($2 million) and Florida billionaire William Koch, who gave $1 million through one of his corporations.