Preckwinkle, who many opponents of Mayor Rahm Emanuel had hoped would run against the mayor, said Arena had been a "strong and consistent ally."
"I support his commitment to protect seniors’ benefits, strengthen public schools and be an ethical and transparent public servant,” Preckwinkle said in a statement.
Arena said he was honored by Preckwinkle's endorsement.
“I look forward to continuing to work with her and elected officials at all levels of government to improve our neighborhoods here in the 45th Ward and tackle the issues facing the City of Chicago,” Arena said.
Arena is the alderman who most frequently votes against Emanuel's proposals, according to a study released earlier this year.
Arena, a founding member of the Chicago City Council's Progressive Reform Caucus, faces three challengers in his bid for re-election on Tuesday.
Also Tuesday, former Vermont Gov. Howard Dean endorsed Arena, saying Chicagoans deserved leaders like Arena "who don't shrink from a fight," according to a statement.
Arena "is committed to Democrats' progressive educational priorities," Dean said, adding the alderman has also been "a steadfast champion for all Chicago students."
In addition, Dean's organization, Democracy for America, said it would spend an additional $60,000 to help Arena win re-election.
The group led by Dean, a former Democratic National Committee chairman, has already spent $50,000 to help the re-election campaigns of seven members of the Progressive Caucus.
A group that supports Emanuel's push for more charter schools has spent $65,400 in an effort to stop Arena from being re-elected, records show.
The other candidates are Chicago Police Lt. John Garrido, whom Arena defeated by 30 votes in 2011, Michelle Baert, who publishes a website and Facebook page filled with listings for family-friendly activities as the 45th Ward Mom, and Michael S. Diaz, who works as an attorney for the state department that regulates banks.
Arena's huge financial advantage over the other candidates in the race has only increased in recent days, records show.
The Service Employee International Union Illinois Council PAC spent an additional $41,500 on Tuesday Feb. 17 to boost Arena campaign through independent expenditures that by law can not be coordinated with the candidate or his representatives, records show.
SEIU has spent $139,700 to send out nearly a dozen fliers in recent weeks to tout Arena's record of "standing up" to Emanuel and tag two of his opponents as Republicans, records show.
Garrido, who got $20,000 from the Fraternal Order of Police PAC on Feb. 6, got another $20,000 from the group on Feb. 10, records show.
Early voting is available through Saturday.
In aldermanic races where no candidate earns 50 percent of the votes cast on Tuesday, a runoff between the top two candidates will take place April 7.
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