The power of the purse is a practical constraint
Congress authorises programmes and appropriates federal money. An administration cannot lawfully spend public funds simply because the president prefers a different allocation; spending authority operates through constitutional and statutory rules.
Budgetary control can therefore become a direct bargaining mechanism between Congress and the executive.
Oversight exposes executive action to institutional scrutiny
Congressional committees hold hearings, request information and conduct investigations as part of the legislature's constitutional functions. The effectiveness of oversight depends on legal authority, enforcement mechanisms and political incentives.
The Senate also exercises advice-and-consent functions over specified appointments and treaties, giving one chamber particular leverage over parts of executive staffing and foreign policy.
Checks are powerful but not automatic
Presidential vetoes, partisan alignment, litigation and supermajority requirements can all make congressional constraints difficult to use. Formal authority therefore does not guarantee practical control.
The balance changes with the issue, the statute and the political composition of Congress.