New York (CNN) A group of powerful Republican donors are hosting two fundraisers in New York City next month to help a dozen sitting senators -- including Arizona Sen. John McCain -- that they say face some of the toughest reelection fights next year.

According to invitations circulated by billionaire investor and major GOP donor Paul Singer and obtained by CNN on Friday, a June 22 "roundtable luncheon" is set to benefit McCain. The former presidential candidate and veteran senator looks increasingly likely to face a primary challenger from the right.

"In the Senate, he has led the fight to strengthen America's national security and our armed forces, eliminate wasteful government spending, and reform government to make it more responsive," Singer, well-known for his hawkish national security policies, wrote in an email invitation.

Another luncheon on June 15 will raise money for five GOP senators that Singer referred to as "some of our most vulnerable Republicans this cycle": Sens. Richard Burr of North Carolina, Ron Johnson of Wisconsin, Mark Kirk of Illinois, Rob Portman of Ohio and Pat Toomey of Pennsylvania.

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Both fundraisers ask guests to give up to $5,400 per candidate -- $2,700 each for the primary and general election.

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