As per the structural theory, the President is weaker in achieving legislation due to the separation of powers principle enforced by the Constitution, in contrast to the fusion of powers in the UK. The US President is unable to sit in the legislative branch, as required by the Constitution to limit the power of any individual. This was seen when Obama was forced to leave his seat in 2009 after winning the Presidential election. In contrast, the UK allows for fusion of powers, meaning the UK PM sits in Parliament, in which he is less reliant on passing legislation, as seen by Boris Johnson, who was dominant in regard to Covid policy. Thus, the structural theory shows that the US President is less effective due to the SOP principle, meaning he has far less influence over legislation, while the PM is able to dominate legislation as he sits in Parliament.

In proposing legislation, the US President is limited by the greater powers of Congress, while the PM enjoys a less autonomous legislature as per the party whip. The US President must ask Congress to pass legislation on his behalf, which is then open to scrutiny by members of Congress. Obama was frustrated in the significant compromises made to the Affordable Care Act 2010. However, in the UK, the PM is the leader of the winning party, which is often a majority, meaning he can whip MPs to tow the party line, as seen during policies on Covid and Brexit. Thus, the rational theory can be seen to make the President less effective as members of Congress are more inclined to act independently of their party, as frequent elections mean they must represent their constituents or face scrutiny and accountability in election time. For MPs in the UK, they must toe the party line; if they don't, they risk progress within their political careers. This means the PM has more freedom, especially during a majority government.

Lastly, the greater power of the judiciary in the US means the President has greater limits on legislation passed than the PM. The President's actions are subject to checks to ensure laws are constitutional, and if unconstitutional, they can be annulled by SCOTUS. For example, Obama's executive orders on immigration were repealed in the US vs. Texas 2016 case. While in the UK, a lack of a codified Constitution means there is less power of enforcement for the judiciary. As seen by Blair's laws on indefinite imprisonment of suspect terrorists only being limited to 28 days after the Belmarsh case. Thus, the US President is less effective as SCOTUS is more powerful in limiting laws than the UKSC is in limiting the PM's policy.