I am a Keynesian - I believe in government stimulus - but I cannot endorse Keynesian policies at this time because we have failed to adhere to Keynesian theory for so long. Put into the simplest of terms Keynesian economic theory tells us that government should engage in deficit spending during economic downturns in order to compensate for lax private sector activity and to spur economic growth. This can be accomplished via spending, tax cuts, or both. This aspect of Keynesian policy has been standard practice since the 1930s. Even the Reagan era policies - often derided as supply side or trickle down - were truly Keynesian. The tax cuts were financed via large federal deficits. These government financed tax cuts spurred tremendous economic growth during the 1980s.
But deficit spending only tells half the Keynesian tale. The rest of the theory holds that during times of economic growth, government must pay down the debt incurred to bring about that growth. So once an economy returns to healthy growth deficit spending must be reigned in, debt paid off. This is accomplished via spending cuts, tax increases, or a bit of both. This aspect of Keynesian economic policy has never been standard practice - rather our government (and in fact most western democracies) have instead maintained the "stimulus" policies of deficit spending even during strong economic times.
If the ratio of debt to GDP continues to rise, lenders may become concerned about the financial solvency of the government and demand higher interest rates to compensate for the increasing riskiness of holding government debt. Eventually, if the debt-to-GDP ratio keeps increasing and the budget outlook does not improve, both foreign and domestic lenders may not provide enough funds for the government to meet its obligations. By then, whether the government resolves the fiscal crisis by printing money, raising taxes, cutting spending, or going into default, economic growth will be seriously disrupted.