An alternative broad-scale explanation links the change in Congress' role to a shift in the substance of foreign policy in an interdependent world where the distinction between foreign and domestic politics is less valid that ever before. The ability of Congress to perform its foreign policy tasks coherently continues to be open to question. Explanations would leave room for major short-term variations in the rate and kind of congressional participation in foreign policy, depending on the specific circumstances and on the individual occupying the White House. The Constitution's shared powers create an "invitation to struggle" for control over foreign policy, and some foreign policy issues become the subject of institutional confrontation. There are fewer questions about the legitimacy of Congress' deep involvement in foreign policy than there are about how well Congress performs its foreign policy responsibilities: efficacy is a bigger issue than legitimacy.