CHICAGO (AP) – Illinois Republican Gov. Bruce Rauner has contributed $50 million to his own campaign fund as he prepares to seek re-election in 2018.

The donation from the multimillionaire former private equity investor was made public in documents filed Thursday. Republicans say Rauner’s contribution is intended as a warning shot as several Democrats weigh whether to challenge him. They include Chris Kennedy, the son of the late Robert F. and Ethel Kennedy, billionaire businessman J.B. Pritzker and state Sen. Andy Manar.

Campaign finance experts say the contribution appeared to be the largest single donation any candidate for state office has given their own campaign.

Rauner spent close to $30 million from his personal fortune and raised another $40 million to win his first public office in 2014.