Robert Hugin, a Republican pharmaceutical executive who has supported former New Jersey Gov. Chris Christie and President Trump, emerged Monday as the first serious challenger to incumbent U.S. Sen. Bob Menendez.

Hugin is prepared to spend at least $20 million in the campaign, according to a person familiar with his thinking.

That would be the most money spent by a self-funded candidate in a New Jersey Senate race since 2000, when Democrat Jon S. Corzine spent more than $80 million, adjusting for inflation.

Menendez, whose approval ratings plunged as he was under the cloud of a long-running federal investigation, hasn't officially announced his reelection campaign. But with the probe over, his top adviser has said there's a "100 percent" chance he will run.

Federal prosecutors, who initially planned to retry Menendez on bribery charges, said on Jan. 31 that they would drop the case after a judge acquitted the senator of some of the charges. A jury sitting in Newark, N.J., failed to reach a verdict during the first trial, and the judge declared a mistrial in November.

Menendez, 64, had $4.1 million in his campaign account as of Dec. 31.

Hugin, 63, retired late last month as executive chairman of Celgene Corp., a global biopharmaceutical company based in Summit, N.J. A Princeton graduate and Marine Corps veteran who grew up in Union City, he joined the firm in 1999 as chief financial officer and was chief executive from 2010 to 2016.

He is expected to launch his campaign formally on Tuesday with stops in North Jersey and Delran, Burlington County.

Hugin has donated about $2.1 million to state and federal campaigns and committees, most of them affiliated with Republicans, since 2007, public records show.

That includes $250,000 in contributions to a super PAC that supported Christie's unsuccessful 2016 presidential campaign, and more than $100,000 to Trump-affiliated committees.

Hugin also has given to some Democrats — including $4,800 to Menendez in 2010, according to records filed with the Federal Election Commission.

New Jersey voters haven't elected a Republican to the U.S. Senate in more than 40 years.