The ability of the president to influence public policy when Congress is in session remains substantial, although there is no question it is greater when Congress is not in session.
Three main aspects of this issue involve the process by which individuals are nominated by the president for public office, but who must be confirmed by the Senate, Congress' control of the federal budget, and the use of executive authorities that circumvent the process by which laws are made.
The president, as Chief Executive, has a plethora of large federal agencies to execute his or her agenda. The Departments of the Treasury, State, Defense, Labor, and so on, all exist to carry out various aspects of the president's agenda, albeit with checks on those authorites by virtue of Congress' influence over the budgets for each of those agencies. Putting aside for the moment the budget issue, however, in order for the president to execute his or her agenda, the top level positions within each agency need to be filled with individuals who agree with the president's agenda. Those top level positions are filled when the president nominates somebody for the job in question, and the Senate confirms that individual.
When one or more senators object to the nomination of a particular individual, they can block the confirmation process by placing a "hold" on the nomination. That hold can continue indefinitely, frustrating the president. However, when Congress adjourns for recess, for example, during the month of August, presidents have been known to use what are known as "recess appointments." These appointments allow for the nominee to begin working the job for which he or she has been nominated but not confirmed.
Another way in which the president can execute his or her agenda is through the use of Executive Orders. Mainly used in matters involving national security, a president can sign a document outlining a policy, with prohibitions on certain activities by U.S. citizens or, conversely, with provisions designed to directly affect the outcome of a crisis or other lingering foreign policy problem. Executive Orders carry the weight of law, even though they are not laws in the conventional sense of being passed by Congress.
Finally, the president's freedom of action is often constrained through Congress' authority over the federal budget. At the beginning of each year, the White House submits a large budget proposal to Congress to fund the agencies that comprise the Executive Branch. The key word here is "proposal." Congress alone has the authority to determine how much money goes to each agency, and how that money is used. Congress literally attempts to micromanage the budget through the passage of laws containing budget figures. The president's ability to circumvent congressional restrictions or earmarks is always a source of conflict between the two branches of government. When Congress is in recess, the president has more freedom to maneuver within those constraints because there is nobody in the Capitol building to stop him or her.
To some degree, this change has meant that the president’s power to influence public policy has been lessened. There are at least two reasons for this.
First, with Congress meeting full time, that body is much more able to put out proposed legislation and to essentially set its own agenda. When Congress met less often, the members of Congress and their staff had less time to work on proposing and passing legislation. This meant that the president had less competition when it came to creating legislation and shaping Congress’s agenda.
Second, with Congress meeting full time, the president has more competition for the public eye. In times past, the president might have been able to use the “bully pulpit” to push for his agenda without having to worry about Congress taking issue with what he said. Congress was not in session so it was much harder for members of Congress to play a prominent role in shaping public opinion. Nowadays, with Congress in session so often (and with so much more media), it is always possible to get members of the opposition party to attack the ideas that the president puts out when he speaks.
In these ways, the president has more competition from Congress today. This reduces his (or someday her) ability to shape public policy.