The conservative outside-spending group Crossroads GPS is launching a fresh wave of ads in four Senate races, including one state that hasn’t been on the organization’s target list all year: Florida.

Crossroads is putting $4.2 million into its latest offensive, a strategist for the group said. In addition to Florida, the ads will hit campaigns in Ohio, Montana and New Mexico.

The spot in Florida is important for several reasons. It’s the first Crossroads advertising aimed at Democratic Sen. Bill Nelson since the debt ceiling fight a year ago. And the commercial focuses on what could be the most explosive issue of the Florida campaign: Medicare.

“Florida’s seniors are facing a health care crisis,” the anti-Nelson ad begins. “But in Washington, Bill Nelson’s been voting for the new health care law, which cuts Medicare spending by $700 billion, puts unelected bureaucrats between Florida’s three million Medicare recipients and the care they need.”

Nelson is facing a challenge from GOP Rep. Connie Mack, who appears to be viable in public polling but whose performance as a candidate has not inspired the strong confidence of national Republicans. The Crossroads ads may help test just how competitive the Nelson-Mack race is.

The ability of Republicans to stretch the Senate map is a question of new significance this week, thanks to the party’s woes in the once-promising Missouri Senate race. If Republicans are unable to mount a strong challenge against Sen. Claire McCaskill, their chances of capturing the Senate may hinge on outside groups’ ability to spend enough money that states like Florida, Hawaii, Maine and Michigan become more winnable.

Two of the races where Crossroads is advertising have long been coveted Republican takeover targets. In Ohio, the group is blasting Democratic Sen. Sherrod Brown over the health care law, while in Montana it’s criticizing Sen. Jon Tester on spending and debt.

The fourth targeted contest, in New Mexico, is a more Florida-like race where Republicans are not entirely sure how realistic a shot they have. Crossroads has mostly run positive ads in the state boosting Republican nominee Heather Wilson. This time, GPS is up with a spot going after Democratic Rep. Martin Heinrich on spending and branding him, alliteratively, “More Money Martin.”

“Big Washington spending is not helping New Mexico. And the more money Martin Heinrich is spending is part of the problem,” the ad says.