Heading into the last month of Nebraska's U.S. Senate campaign, Republican challenger Pete Ricketts had three times as much cash on hand as Democratic incumbent Ben Nelson.

According to the candidates' most recent federal filings detailing spending, Ricketts had $1.67 million cash on hand as of Sept. 30. Nelson had $449,213.

The race is now the most expensive campaign of its kind in state history, with the two having spent more than $15.85 million.

The previous spending record was set in 1988 when Bob Kerrey, a Democrat and former governor, defeated Republican U.S. Sen. David Karnes in a race where the candidates spent a combined $7 million.

That would be equal to an estimated $11.48 million today, based on the Consumer Price Index, which gauges the price of goods and services bought by the public.

For the July-through-September reporting period, Nelson generated almost five times the amount of campaign contributions as Ricketts.

Ricketts, fueled by his personal fortune, continues to outspend Nelson by a wide margin.

Nelson's filing, released Saturday, showed that Nelson had received $1.18 million in contributions during the three-month period, raising his total to $6.93 million.

Ricketts, whose filing was made public Friday, raised $250,671 over the same period, increasing his total to $1.34 million.

But Ricketts has pumped $9.61 million of his own money into his campaign, including $4.2 million during the July-through-September period.

The former TD Ameritrade executive has said he's worth $45 million to $50 million.

Nelson has not used any of his own money.

For the campaign, Ricketts has spent $9.29 million and Nelson $6.56 million.