In 1870, Brown had been elected governor of Missouri on a Liberal Republican platform of universal suffrage (including for women), universal amnesty for former Confederates, civil service reform, lower tariffs, and an eight-hour day.  Missouri Democrats also endorsed his candidacy, thereby providing a blueprint for the national coalition of Liberal Republicans and Democrats two years later.  In January 1872, Senator Carl Schurz of Missouri, judging the Grant administration to be irredeemable, issued a call for a national convention of liberals to nominate a candidate for president.  Over the next few months, Liberal Republican editors, delegates, and other supporters warned repeatedly that caution should be taken in choosing the best nominee.  

Governor Brown was Missouri’s favorite-son candidate, but he lacked broad appeal.  The top presidential contenders were former diplomat Charles Francis Adams and Justice David Davis of the U.S. Supreme Court.  Horace Greeley, the maverick editor of the New York Tribune, had some support but was considered more as a possible vice-presidential nominee.  On the first ballot at the Cincinnati convention, Adams made a strong showing and Greeley finished competitively in second place.  At that point, Governor Brown promptly withdrew and endorsed Greeley, in part to block Adams, the candidate of Brown’s sworn enemy in Missouri, Senator Schurz.  That gave momentum to Greeley, who won the nomination on the sixth ballot, after which the nominee's floor managers urged the convention to select Brown for the second spot on the ticket.

This cartoon (published May 8) was Thomas Nast's first since the Liberal Republican National Convention concluded on May 2.  The artist had assumed that Adams would be nominated for president, and had nearly completed this drawing when he learned of the surprise selection of Greeley.  Nast altered the picture, probably on the woodblock after it had been engraved, by changing the tiny head on the mouse from Adams's face to Greeley's and by inscribing Brown's name on the tail. (This is why Greeley appears twice, watching his own figure as a mouse).  The caption about a mouse issuing forth from a mountain mimics the story of two Aesop's fables (later consolidated by Phaedrus), a common source for Nast.  The cartoonist may also have been attracted to the imagery by recent news reports of the explosion of Vesuvius, the Italian volcano, on April 12, 1872.  

The Liberal Republicans staring at the emergent mouse are (left to right):  Senator Reuben Fenton of New York; Greeley; Senator Lyman Trumbull of Illinois; Senator Carl Schurz of Missouri, in whose hat a paper indicates his desire to be secretary of state; and a blathering Senator Thomas Tipton of Nebraska, holding his speeches about Tom, Dick, and Harry.  In the right background are (left to right):  Frank Blair, the Democratic vice-presidential nominee in 1868, and August Belmont, the chairman of the Democratic National Committee.  The Democrats, lacking viable candidates of their own, would endorse the Liberal Republican ticket within a few weeks.