Grassley will be a "special guest" at two fundraisers for David Young (R), who was his chief of staff until late May, according to invitations obtained by the Des Moines Register.

The longtime senator told The Hill in early June that he wouldn't endorse in the race, though he voiced concern that the eventual GOP candidate would be short on money against Rep. Bruce BraleyBruce Lowell BraleyOPINION | Tax reform, not Trump-McConnell feuds, will make 2018 a win for GOP Ten years later, House Dems reunite and look forward Trump: Ernst wanted 'more seasoning' before entertaining VP offer MORE (D-Iowa), Democrats' chosen candidate for the seat who won't face an expensive primary.

The move likely pits Grassley against Iowa Gov. Terry Branstad (R) in the Iowa Senate primary, as Branstad has quietly pushed Iowa state Sen. Joni Ernst (R) to run for the seat.

Both men have been careful to stay publicly neutral in the race to replace retiring Sen. Tom HarkinThomas (Tom) Richard HarkinDem Senator open to bid from the left in 2020 Senate GOP rejects Trump’s call to go big on gun legislation Trump should require federal contractors to follow the law MORE (D-Iowa), but as the race heats up, it will be interesting to see how involved each gets for their preferred candidate. Former U.S. Attorney Matt Whitaker (R) is also in the race.