McMahon defeated former U.S. Rep. Christopher Shays in Tuesday's GOP primary.
It's the second time the state's Republicans have chosen McMahon over a veteran politician as the standard-bearer in the general election for an open Senate seat. McMahon lost in 2010 to then-Attorney General Richard Blumenthal, a Democrat, after spending about $50 million of her own money. She has loaned or given about $15.7 million to this campaign so far.
Shays, a moderate Republican who represented the 4th Congressional District from 1987 until he lost in 2008, had touted his Washington experience during the primary campaign.
The seat is being vacated by retiring Sen. Joe Lieberman, an independent.