(Bloomberg) -- Democrats are courting progressive-minded Americans by calling for a tax on Wall Street trades. If the party succeeds, the mom-and-pop investors they're wooing could bear the brunt.

The party supports a financial transaction tax to rein in "excessive speculation" and fast traders that have "threatened financial markets." Small investors would probably see an increase in the cost to buy and sell, which has never been lower. The tax, which is aimed at the speed traders that are Wall Street's dominant middlemen, could also raise trading costs by reducing liquidity if those firms retreat because of lower profit margins.

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