Voters in North Carolina, West Virginia, Indiana and Wisconsin are heading to the polls Tuesday.
The race for the GOP nomination is now a mere formality and the focus now shifts.
Longtime Indiana Senator Dick Lugar faces an uphill battle against a conservative challenger.
In North Carolina, voters will decide on a constitutional amendment defining marriage between a man and a woman, banning same sex marriage.
And voters in Wisconsin will head to the polls to decide who will challenge Governor Scott Walker in a recall election next month.
And the Senate is expected to take up a plan to keep student loan rates from doubling on July 1.
House Republicans passed a one year extension last week, paying for it by cutting from a preventive healthcare fund.
The Senate Democrats' plan would pay for the six billion dollar extension by collecting more social security and medicare payroll taxes from the wealthy.
Both bills freeze student loan interest rates at 3.4%.