Latent Political Risk Undermines Bond Optimism

After a short respite in early February, the U.S. bond market's yield curve has begun to flatten again. This bears watching because a shrinking gap between short- and long-term yields is generally associated with the outlook for slower economic growth. But given the tax reform and fiscal stimulus, why is the curve narrowing again in maturities beyond two years? I see two explanations:

Most market participants are skeptical that the recent fiscal stimulus measures, including the tax reform bill, will have enough supply-side impact to come close to offsetting the costs. There are groups of growth-at-any cost Republicans and Democrats in Congress, but they are a minority in each party and rarely vote together, so they do not have true political leverage.