HARRISBURG, Pa.—One of the five Republicans seeking to challenge Democratic U.S. Sen. Bob Casey spent almost $1 million per month, mostly on TV advertisements, heading into the last stretch of the primary race, his campaign reported this week.

Tom Smith reported almost $2 million in his campaign account as of April 4 after spending nearly $3 million in the first three months of 2012. He lent his campaign most of that money, but he also outraised a key rival, Steve Welch, about 2-to-1 in outside contributions during the period.

Welch, who is also airing TV ads, had about $390,000 left after spending about $770,000 in the period. He has invested $1 million of his own money.

On Friday, Welch attended a fundraiser at Lehigh University headlined by Gov. Tom Corbett.

The primary is April 24. Welch and Smith have raised and spent far more money than the other three Republican candidates combined. They are David Christian, Sam Rohrer and Marc Scaringi.