So here is the thing. The US economy has not been at the zero-lower bound for more than 2 years now, meaning that the Federal Reserve is determining aggregate demand. Any stimulative fiscal policy will simply be offset one to one by higher interest rates and tighter monetary policy, thus leaving total demand roughly unchanged.
It also should be noted that the Trump tax cuts were simply a big handout to the rich who have a lower marginal propensity to consume anyway. While corporations have increased their stock buybacks in recent month, the impact on corporate investment seems to have been extremely limited. Most studies assume that the supply side effects of the Trump tax cut will be extremely limited while the Fed will also mostly offset any potentially stimulative impact on aggregate demand.
All of this means that the Trump tax cuts will most likely blow a huge while in the budget deficit in the years to come. While the American fiscal position is not very dire, some projections show that the debt to GDP ratio is approaching 100% over the next few years. Ultimately, the current tax cuts will have to be paid for with a combination of lower spending or higher taxes in the future. While a Keynesian stimulus would have been appropriate for a number of years right after the crisis when unemployment was high and above the natural rate while interest rates were still at zero, the poorly designed tax cuts right. now could not have been come at worse time. As inequality has been surging anyways for many years (mostly the result of rising asset prices like housing while globalization might have put downward pressure on unskilled labor in advanced economies), handouts for the rich without any impact on aggregate demand is just the last thing the US economy needed.
As for Italy, the country's debt to GDP is already exceeding some 130%. One of the main problems is that Italy's GDP has been stagnating for almost 2 decades now, with real incomes barely exceeding the ones when the country joined the Eurozone.
While Italy surely suffers from many structural problems, there is a case to be made that some of the country's problems are a result of the Euro. Italy's real exchange rate is simply too high relative to its trading partners. The tight monetary policy stance that the ECB adopted from 2008 to about 2014 surely did not help either.
Regardless, the new populist Italian government will not do anything to improve the country's economic performance. Looting the retirement age is a costly economic handout in a country that is ageing rapidly and will simply blow a huge hole in the country's budget. Bond markets are tightly worried. The election result have put significant downward pressure on the Euro while Italian long-term bonds have increased by more than 40 basis points relative to German bonds in just a few weeks. In Italy's case, the bond market vigilantes are actually watching (even though there is a case to be made that the recent sell off is overdone).
Both the Trump tax cuts as well as policy proposals that have been put forward by the Italian government will do significant long-term damage to the fiscal sustainability. In the US, the stimulus could not have come at a worse time given that the demand impact will be offset with tighter monetary policy. That fiscal space will be lacking in a future recession, which is especially worrisome given that interest rates are still relatively low and monetary policy might not be able to offset a future aggregate demand shock.
In the case of Italy, the country is already facing large debt sustainability issues. While most of the problems might be the result of adopting the common currency, populist handouts will do more damage than good. Just like in the 1930s, populism/fascism will do a lot of damage to a lot of people. Let's see how and where it will stop this time.
PS: Trump is not a populist. His recent remarks as well as all the other remarks on people of different color and ethnicity make it abundantly clear that a racist is sitting in the Oval Office at the moment. The Trump administration clearly displays fascist tendencies. We should not confuse it with populism.