EXHIBIT 31.2

Certification Pursuant to Section 302
of the Sarbanes-Oxley Act of 2002

I, Michael P. McGrath, Vice President
and Chief Financial Officer of the registrant, certify that:

|1. | I have reviewed this annual
      report on Form 10-K of Mercantile Bancorp, Inc.;                                                                                                                                                                                                                                                
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|2. | Based on my knowledge, this
      report does not contain any untrue statement of a material fact or omit to
      state a material fact necessary to make the statements made, in light of
      the circumstances under which such statements were made, not misleading
      with respect to the period covered by this report;
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|3. | Based on my knowledge, the
      financial statements, and other financial information included in this
      report, fairly present in all material respects the financial condition,
      results of operations and cash flows of the registrant as of, and for, the
      periods presented in this report;                   
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|4. | The registrants other certifying
      officer(s) and I are responsible for establishing and maintaining
      disclosure controls and procedures (as defined in Exchange Act Rules
      13a-15(e) and 15d-15(e)) for the registrant and have:                                                                                  
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|   | (a)                                                                                                                                                                                                                                                                                                                               
|  
|   | (b)                                                                                                                                                                                                                                                                                                                               
|  
|   | (c)                                                                                                                                                                                                                                                                                                                               
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|5. | The registrants other certifying
      officer(s) and I have disclosed, based on our most recent evaluation of
      internal control over financial reporting, to the registrants auditors
      and the audit committee of the registrants board of directors (or persons
      performing the equivalent functions):        
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|   | (a)                                                                                                                                                                                                                                                                                                                               
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|   | (b)                                                                                                                                                                                                                                                                                                                               

Date: March 28, 2006

|/s/                                          | MICHAEL P.
      McGRATH
|-------------------------------------------- | ------------------------
|MICHAELP. MCGRATH                           
|Vice President and Chief Financial
  Officer

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