Final answer:
The ultimate weapon in the President's arsenal to influence Congress is the mobilization of the public, aided by media and social media attention, as well as the strategic use of interest groups and the presidential office's own persuasive tools like the 'bully pulpit.'
Step-by-step explanation:
The ultimate weapon in the President's arsenal of resources to influence Congress is likely the mobilization of the public. Modern presidents may find more success in using their popularity to increase media and social media attention on an issue. Even if the president is not the reason for congressional action, they can cause the attention that leads to change.
Interest groups support candidates sympathetic to their views in hopes of gaining access to them once they are in office. They provide lawmakers with information and cues about how to vote on issues. Interest groups and their lobbyists engage with traditional media and social media to spark attention and use the power of public opinion to their advantage.
Another important persuasive power available to the president is the use of the office and its trappings, such as the "bully pulpit." The president can call the media together to deliver messages, enhancing their ability to influence public opinion and therefore Congress indirectly.