Senate Republicans raised $4.1 million in June as they continue to prepare for what could be a very tough 2016 election.

The NRSC, formerly the National Republican Senatorial Committee, closed the month with $6.5 million in the bank and no debt left over from the 2014 elections. The $4.1 million raised by the committee constituted its most successful June fundraising ever for an off year. Two years ago in June, the NRSC raised only $3.7 million.

The party's seven-month-old, four-seat Senate majority could fall in 2016. The Democrats are targeting at least five vulnerable seats, much as the Republicans targeted — and won — several vulnerable Democratic seats in 2014. The GOP won nine Senate seat last November, rising from a 45-seat minority to a 54-seat majority.

Vulnerable Republicans include Sen. Mark Kirk of Illinois, Sen. Ron Johnson of Wisconsin, Sen. Pat Toomey of Pennsylvania, Sen. Kelly Ayotte of New Hampshire and Sen. Rob Portman of Ohio. The Democrats also are targeting an open seat in Florida, where Sen. Marco Rubio is retiring to run for president.

Republicans are targeting an open seat in Nevada, where Senate Minority Leader Harry Reid, a Democrat, is retiring. June fundraising figures for the Senate Democrats' campaign committee were not yet available.