Democrats are supposedly the ones who "help" the poor. Measured in terms of the fraction of people who are not obligated to pay income tax, hardly any progress was made during the Clinton years. It was two laws passed under the current administration that exempted large numbers of poor from the income tax.
Professor Mankiw reminds us that Democrats renege on campaign promises too (see here for Republican examples): President Clinton changed his mind about "middle class tax cuts."
By the way, the phenomenon that Democrats and Republicans have essentially the same public policies is evidence that our political system is pretty efficient. If Democrats and Republicans actually differed as much as their rhetoric does, public policy would wildly shift every time there was a change in the party in power. Wild policies shifts would be bad for the economy.
If instead Democrats and Republicans were blunt and said "Ultimately, I'll do essentially the same thing as my opponent does" (McCain and Obama are almost this blunt when it comes to policies related to the financial crisis), the political process would lose its value to many people. I might call it "entertainment value," but that sounds pejorative. It's more like the difference between Coke and Pepsi -- they might be chemically indistinguishable, but consumers would not be well served if Coke said "our drinks are essentially like Pepsi's" and Pepsi said "our drinks are essentially like Coke." Indeed, I wrote an earlier post to cheer up the November 4 losers, but it may have the opposite effect because there is value to imagining that your favorite brand is unique. Many people look at the Democrat and Republican brands that way.