It seems such a simple truth but this site discusses the deeper meaning of the story. Check out
Symbolism of the Wizard of Oz if you are interested.
The Cowardly Lion, according to Henry Littlefield, represents William Jennings Bryan, who made the first of three unsuccessful bids for the presidency in the election of 1896. Bryan was the Democratic Party's nominee for president and he embraced some Populist issues, most notably "free silver," the bi-metallic monetary standard that Populists thought would allow farmers greater access to credit. The Populists were faced with the choice of either running their own candidate or choosing "fusion" with the Democratic Party. The Populists opted to select Bryan as their candidate as well, risking being absorbed by the Democratic Party and dwindling as a movement. By casting their lot with the Democrats, the Populists felt they could expand their influence outside of the rural parts of the country. Ultimately, however, it appears the Populists made the wrong bet. Bryan lost the election and the Populists were never regained the influence they enjoyed in the 1890s. Perhaps the undoing of the Populist movement was their failure to attract the support of industrial workers. In Baum's original story, upon first meeting the Cowardly Lion strikes at the Tin Man, but his claws do not make a dent in his metal body—just as the Populists efforts to create a coalition between farmers and industrial workers were unsuccessful. Littlefield equates this futile act on the part of the Cowardly Lion with Bryan's failure to win the vote of industrial laborers. Littlefield suggested that Baum revealed his skepticism about politicians through the character of the Cowardly Lion. Bryan may have been a great orator, but despite his roar, he had no real power.
Gene Clanton agrees that the Cowardly Lion could represent Bryan but he sees a broader meaning in this symbol. He interprets the "Cowardly Lion as William Jennings Bryan or any major party politico cowed by the money power."  In The Historian's Wizard of Oz, R. Dighe includes editorial cartoons from the 1890s that depict William Jennings Bryan as a lion. He points out, however, that the image of a lion was also used for the Populist Party in political cartoons, so perhaps the Cowardly Lion represents not an individual, but the Populist Party generally.