Though Booker's policy stands are mostly favorable to the left, he's faced withering criticism from some of its loudest voices over his coziness with Wall Street.

Securities and investment firms have given Booker about $1.88 million of his total $17.6 million in contributions this election cycle, according to records from SNL Financial and OpenSecrets.org. The industry has been the biggest group to give to the former Newark mayor, with lawyers and real estate firms the two next-highest.

An industry that has historically favored Republicans is spreading out campaign contributions this fall in an attempt to influence Washington lawmakers on the plethora of banking reforms already enacted or in the pipeline.

Financial firms are shelling out big cash for the mid-term Senate elections, but their favorite candidate is an unlikely one: left-leaning New Jersey Democrat Cory Booker .

In an analysis published more than a year ago, The Atlantic wondered "Why do liberals hate Cory Booker?" while the Daily Beast a few days ago published a takedown focused on the "ugly truth about Cory Booker."

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Booker's appeal to Wall Street is probably geographic as much as anything: Many financial firms are based in New Jersey, and he's spoken in the past with at least some degree of antipathy toward the way some Democrats demonize Wall Street.

Booker's campaign did not immediately respond to a request for comment.

Among securities and investment firms, the next most-popular candidate has been Senate Minority Leader Mitch McConnell, the Kentucky Republican who has pulled in $1.27 million. In fact, cash distribution among the sector has been fairly balanced, with six of the top 10 recipients being Republican, according to SNL.

Read MoreHedge funders love Booker, too

In other Wall Street-related industries, McConnell leads in the insurance sector, with $520,525. Insurers have a slightly stronger Republican bias, with the GOP taking four of the top five and seven of the top 10 positions. However, the top 20 is even split between the parties.

In the finance/credit group, McConnell again is the leader with $86,500 while Booker is third with $63,800.

Wall Street buys GOP's affections with campaign cash

The campaign moves come as Washington deliberates measures that affect banking, and financial services firms try to move the chips in their favor. As an SNL analysis puts it:

The legislative window in 2015 is expected to be tight, with policy discussion cut short by political maneuvering ahead of the 2016 presidential race. That means Wall Street's most ambitious goals, such as new limits on the Financial Stability Oversight Council and Consumer Financial Protection Bureau, or tax reform favoring lower corporate tax rates, might not make it into law. Yet industry groups believe there is still room to move those issues closer to the finish line, even if the initial benefits are indirect.

A look at the leaders among the major Wall Street/finance sectors: