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CHICAGO (AP) — Illinois Gov. Bruce Rauner contributed $50 million to his own campaign fund as he prepares to seek re-election in 2018 — a record-setting amount that Republicans said was intended as a warning to Democrats considering challenging him.

The money from the multimillionaire former private equity investor, made public in documents filed Thursday, appeared to be the largest single donation any candidate for state office has given his or her own campaign, campaign finance experts said. Rauner's team indicated there was more to come, describing it as a "first installment."

The contribution comes as several Democrats are weighing whether to take him on. Among them is Chris Kennedy, the son of the late Robert F. and Ethel Kennedy and former operator of Chicago's Merchandise Mart, who's expected to make an announcement next month.

Billionaire businessman J.B. Pritzker, state Sen. Andy Manar and U.S. Rep. Cheri Bustos also have said they are considering bids.

The Illinois GOP, funded almost entirely by Rauner, already has been attacking potential candidates in online ads and email blasts.