If they are so worried about the "a job-killing tax hike" discussed in their letter (a distinctly Keynesian perspective by the way), then use the money gained from allowing the tax cuts to expire for the wealthy to fund an extension of the Making Work Pay tax cuts in the stimulus package that went to middle and lower class households. The GOP is refusing to extend the Making Work Pay tax cuts, apparently the jobs lost when taxes go up for the non-wealthy don't count. Since these tax cuts are likely to result in more spending than tax cuts for the wealthy, this would increase rather than decrease jobs.
Mark is basically noting that the marginal propensity to consume for lower income groups is likely to be higher than the marginal propensity to consume for higher income groups. But as long as the marginal propensity to consume is less than one, increases in government purchases have a larger bang for the buck than do tax cuts. Yet – most Senate Republicans oppose any increase in government spending on the grounds that it might increase the deficit. It might be a refreshing change if McConnell actually adopted the perspective of a Keynesian economist – but to date, his tone seems to be one of a Machiavellian whose quest of power is motivated so that he and his ilk can shift more income towards the very rich.