Sen. Johnson - who was elected in 2010 with strong Tea Party backing - joined fellow Badger State lawmaker Rep. Paul Ryan (R-WI) in backing Romney's campaign for the White House, which seems to be on the verge of a triple win Tuesday in primaries in Maryland, Washington, D.C. and Wisconsin.
With Romney heavily favored in Maryland and D.C., Wisconsin has become the battleground in recent days, with Romney and Santorum totally focused on the Badger State, and then will continue today as well.
The last four polls have Romney ahead of Santorum by ten, seven, five and eight points, with Newt Gingrich and Ron Paul not even a factor, struggling to get into double digits.
Over the weekend, Santorum vowed to go on in the GOP race even if he loses all three primaries on Tuesday - but realistically, his path forward is getting more and more difficult.
The only other primaries in April are in Pennsylvania, New York, Delaware, Connecticut and Rhode Island. Pennsylvania is Santorum's home state - but the others would seem to be prime turf for Romney.
And every delegate that he chalks up this month makes it that much more difficult for Santorum to stop Romney's drive for the GOP nomination.