WASHINGTON — The 2012 presidential election broke the $2 billion milestone in its final weeks, becoming the most expensive in American political history, according to final federal finance reports released Thursday. The reports detailed a last-minute cascade of money from mega-donors and an onslaught of spending by the Obama and Romney campaigns and “super” political action committees.

The final campaign finance tallies filed with the Federal Election Commission included nearly $86 million in fundraising for the losing presidential candidate, Republican Mitt Romney, in the election’s last weeks. That final burst brought the Romney campaign’s total for the election to above $1 billion. Final fundraising and spending totals for President Barack Obama’s victorious drive also topped $1 billion.

Surpassing the $2 billion mark was long expected after an election season dominated by the supercharged competitive pressures both campaigns faced in mounting massive fundraising blitzes to stoke expensive media ad battles and ground wars.