Deficit spending is a clear example of an economic problem-turned-political issue. But long before the current political divide, funding of war efforts was a hot topic of debate among leading economists. Mankiw (p 408) offers the two principal economic arguments debt financing of a war: smooth tax rates over time and shifting the burden to future generations. These arguments look sensible, and more importantly, are an easy "sell" for any politician. Deficit spending is popular but also a historically-demonstrated economic policy. Part of Keynesian Economic Theory, deficit spending during a recession is a counter-cyclical fiscal policy to stem the tides of the business cycle (growth and recession); during booming economic growth periods, increasing taxes and cutting stimulatory spending are meant to slow growth and delay the inevitable recession. According to Keynes, the government must play an active role in managing the economy in the short run and not wait for the 'invisible hand' to steer the economy in the long run. This was because, as Keynes is famously quoted, "In the long run, we are all dead." (SOURCE: Wikipedia.org and associated references.)
Economists have generally agreed with the necessity of a stimulus package, despite differences in professional opinion on the role of government or what the package itself ought to contain. (Source: Washington Post; source used elsewhere in this post as well.) The stimulus package is also based in Keynesian counter-cyclical policy and has historical precedent. The ability of an economic stimulus to actually stimulate the economy in the wake of deficit spending and recession is based on the "Keynesian Multiplier": Developed by Richard F. Kahn in 1931, a monetary outlay 1.) Is spent mostly on consumption and modestly put in savings, and 2.) The spending and investment allows businesses to grow and create jobs. Data from the 2009 stimulus suggests a multiplier of 2.5, with the number dampening to equilibrium (multiplier of 1) over time. The size of the government, and therefore the level of control it has over the economy, plays a key role in this as well. A large government will be able to have a more rapid effect on market outcome but place a larger burden on each citizen to support this, and the burden would be further exacerbated by the fact that the stimulus is increasing a deficit that is already large and growing due to the war effort. The government we have today accounts for almost 50% of the GDP.
The size of the government in and of itself is as large today as it has ever been, save during the WWII era when it spiked to the 50% level but then shruck back to pre-war levels shortly after. Combine this with a large and growing war deficit, a deep recession, bank failures, job losses, and a trade deficit, and the historical precedents no longer add up entirely: The situation has gotten out of the Keynesian Economic hand. The government has the capability to run the private sector market down too far; the debt is so large that future generations aren't just going to pay with taxes, inflation, and higher interest rates, but with major cuts to social welfare programs. A recent CNN Research poll mirrored the new political and economist view that something else must be done besides the stimulus to help the economy- namely, cut government spending now to prevent larger cuts in the future. Americans now overwhelmingly (81%) support cuts to foreign aid programs in order to reduce the deficit; they overwhelmingly (80-85%) are against significant funding cuts for imperiled social programs such as Social Security, Medicare, and Veteran's Benefits. Aligned with popular sentiment, there are budget cutbacks with which most politicians and economists agree (see CNN.Money article). Budget cutbacks are a recessionary measure of Classial Economics theory, which contradicts Keynesian theory in premise. Even without theory, it seems quite contradictory to go borrowing a large amount of money for government outlay and cutting taxes to stimulate the economy, then looking to lower government influence and cut as much spending as possible. Important to note in this case is that Keynesian counter-cyclical policy measures are only meant to be short run. The stimulus is the jab at the recession, looking to increase demand and availability of investment capital to jumpstart growth. As the multiplier reaches equilibrium, and the supply of savings has increased, the government then takes the cross-punch to the recession by shrinking in size and paying the deficit in order to make more savings available to the private sector again as well as preserve social welfare programs for the future.
OTHER SOURCES: http://money.cnn.com/2010/06/03/news/economy/U.S._debt_impact/index.htm?iid=EL&iid=EL
http://faculty.chicagobooth.edu/john.cochrane/research/papers/fiscal2.htm