A wave of last-minute donations fueled massive spending in this year’s presidential race. (John Moore/GETTY IMAGES)

Federal Election Commission reports filed Thursday reveal manic activity in the final days of the 2012 campaign, with a wave of last-minute donations fueling massive spending.

President Obama’s campaign received contributions from 1.3 million donors in the last two weeks of the campaign and spent more than $100 million during that period.

Republican Mitt Romney’s campaign raised $85.9 million during the same period from nearly 600,000 donors. Together, the campaigns spent more than $2 billion, part of a record $6 billion election tab. The totals were fueled this year by “independent” campaign committees that emerged after a 2010 Supreme Court decision permitted unlimited corporate and union spending.

A super PAC backing Romney spent about $45 million in the final two weeks after receiving checks from wealthy donors, including $10 million from Sheldon Adelson, the Las Vegas gambling magnate.

The reports that were available early Thursday evening showed extensive activity by Adelson, who spent an estimated $100 million on campaigns this election cycle. For example, Adelson helped fund a super PAC that formed close to the election and paid for attacks on Sen. Debbie Stabenow (D-Mich.), who easily won reelection.

Thursday’s filings show that Adelson contributed $1 million to the group, which reported spending $1.1 million in opposing Stabenow. Adelson’s role was reported first Thursday by the Center for Responsive Politics, which reviews FEC data.

Adelson, who also owns casinos in Singapore and Macau, provided more than $54 million to Republican presidential candidates this cycle. He gave an additional $18 million to other Republicans. As a result of his largesse, the billionaire is courted by ambitious politicians and conservative causes.

Adelson’s spokesman, Ron Reese, declined to comment Thursday. The casino executive and his wife were in Washington this week for a variety of events, including meetings with officials on Capitol Hill. The spending details revealed Thursday show the growing influence of wealthy individuals in addition to Adelson.

The filings also detailed campaign spending by New York Mayor Michael R. Bloomberg (I). The filings show that Bloomberg was the only donor to the Independence USA PAC, which sent more than $8 million to help candidates who could aid causes he backs, such as environmental action and gun control. FEC reports show that Bloomberg’s biggest expenditure was nearly $3 million on ads attacking Rep. Joe Baca (D-Calif.), who has opposed some legislation restricting guns sales. Baca lost his reelection bid to another Democrat, Gloria Negrete McLeod.

The full story of the 2012 election spending wasn’t immediately clear. Computer problems delayed the receipt of the full Obama FEC report. A campaign spokeswoman, Katie Hogan, said late Thursday that the final report would show that Obama’s joint campaign committee had raised more than $1.1 billion from a record 4.5 million donors.

The final wave of money came mostly in the form of small checks. Hogan said the average amount contributed after Oct. 18 was $41. Results weren’t available late Thursday for spending by a super PAC backing Obama, which collected checks in larger amounts from a relatively small group of donors.