Rep. Jesse Jackson Jr. (D-Ill.) has resigned from Congress amid growing mental health concerns and a federal investigation into his campaign finances.

Jackson, 47, submitted a letter of resignation to Speaker of the House John Boehner, an aide of Boehner's told the Washington Post.

It had been rumored that Jackson would resign, as part of a plea deal with federal law enforcement officials, who were investigating whether he used campaign donations to decorate his home and buy a female acquaintance a $40,000 watch. Investigators also were examining whether Jackson attempted to buy Barack Obama's vacated Senate seat from now-incarcerated ex-governor, Rod Blagojevich, in 2010.

In the last year, Jackson has been in and out of the Mayo Clinic, where he has been receiving treatment for a bipolar disorder. Jackson twice left Mayo facilities without alerting constituents where he was headed. His isolation led Senator Dick Durbin (D-Ill.), and Chicago Mayor Rahm Emmanuel, to question the congressman's dedication to Illinois.

Despite concerns regarding his mental health and commitment to constituents, Jackson was convincingly reelected in November, due in large part, to his “safe” Democratic district. His campaign was run mostly through robocalls, while Jackson remained at the Mayo Clinic.

Jackson now will not be sworn in for the seat. Pat Quinn (D), the governor of Illinois, must now declare a special election to take place within 115 days of Jackson vacating the position.

Candidates for Jackson's old seat include his wife, Chicago Alderwoman Sandi Jackson (D); Cook County Chief Administration Officer Robin Kelly (D); state senator Toi Hutchinson (D); and state senator Napoleon Harris (D), who's also a former NFL player, the Washington Post reported.

Former congressman Mel Reynolds (D-Ill.) has also been mentioned as a replacement. Jackson took Reynolds' seat in a 1995 special election, after Reynolds was convicted on charges of sexual misconduct with a 16-year-old campaign worker.

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