First, you've got to distinguish between tax schemes to increase mass purchasing power in times of deficient demand and "supply side" tax cuts. "Supply side" of course is Reagan-speak for the "revolt of the haves" that began with Proposition 72 in California during the 70s and culminated in the 2000 Bush coup d'etat.
In today's economy, the best example of a Keynesian tax cut proposal would be a holiday on FICA deductions. But even that scheme would offer little help to the jobless, whose spending needs goosing the most.
Another Paul Krugman webpage is quite instructive on the politics and economics of "supply side" tax cuts sold as Keynesian through clever bait and switch.
"SLICING THE SALAMI
SYNOPSIS: The rationales for Bush's tax cut wear away bit by bit
Basically, there are three federal taxes on individuals. The payroll tax, which is levied at a flat rate of 15.3 percent of income up to a maximum of almost $70,000, is the main tax paid by about four out of five families. The income tax is less than 10 percent of income for most families, but it rises to around 30 percent of the income of million-dollar earners. And the inheritance tax, which applies only to estates of more than $675,000 (twice that for couples), is a tax on only the very well off: a mere 2 percent of estates pay any tax, and most of the tax is paid by a few thousand multimillion-dollar estates each year.
Now for the salami tactics.
Conservatives who decry the burden of taxes always include the payroll tax in their calculations. And when arguing for tax cuts, the administration starts with numbers that include the whole salami. Again and again we hear about that projected surplus of $5.6 trillion. You shouldn't believe that projection, but for what it's worth more than half of it (the more credible half) comes from Social Security and Medicare programs financed by payroll taxes.
When it comes to tax cuts, however, Mr. Bush's people ignore the payroll tax that is, they propose no cut in the tax that is most of what most families pay, while demanding a large cut in the income tax, which falls mainly on the affluent. And they want to eliminate the inheritance tax, which is overwhelmingly a tax on the downright wealthy.
By proposing to eliminate a tax that falls entirely on the rich, to cut a tax that falls mainly on the well off, but to ignore the main tax paid by most people, the administration has made a deliberate decision to tilt tax relief strongly toward the top of the scale. Families earning $50,000 per year would on average get a tax break of about $800 annually; families earning $1 million would get about $50,000." From
http://www.stern.nyu.edu/globalmacro/cur_policy/bush_tax_cut_readings.html