Congress is often willing to defer its war-making prerogatives when the president is of the same party as the congressional majority, a new University of Michigan study indicates.

The composition of Congress influences how much anti-war legislation—which impedes the president’s ability to carry out military actions—is introduced, says Michael Heaney, assistant professor of organizational studies and political science.

Democrats led the opposition to the Iraq War in the 107th through the 110th Congresses, leveraging it to win control of Congress in 2006 and the presidency in 2008.

“Yet, when the Democrats were finally positioned to make good on their promises of peace, they abandoned the crusade,” said Heaney, the study’s author. “The election of a Democratic president in 2008 reduced Democratic willingness to back the anti-war cause.”

After a failed round of negotiation with Iraq’s government, the president announced last month that he was pulling remaining U.S. troops from Iraq by the end of this year, though thousands of military contractors will remain. There are currently about 39,000 troops in Iraq. The surge of troops in Afghanistan is expected to dip late next year.

The new study analyzed the link between political parties and war-making powers by Congress and the president from 2001 to 2011. While Democratic partisanship sometimes yields incentives to oppose war and Republican partisanship sometimes encourages support for war, these incentives may be in reversed, in part, depending on the party of the president at the time.

Heaney looked at who sponsored or co-sponsored anti-war bills, as well as variables such as partisanship/ideology, Iraq and Afghanistan, position in Congress and personal characteristics. For the Iraq and Afghanistan variables, it included the number of troop deaths, the percentage of the public against the war (Iraq) and how the politicians voted for the war.

Members of the 110th Congress introduced 76 anti-war bills—more than the 65 bills introduced by the three previous Congresses and the current one combined.

Heaney says that support for anti-war positions significantly increases with the rise in troop casualties or when public opinion turns against war. Congress, he adds, has the power to exert pressure on the president’s conduct of war, such as in Afghanistan, if it chooses to do so.

“Congress’s decision to use its constitutionally granted war-making powers could make all the difference in whether American troops leave Afghanistan soon or if the occupation continues to linger for a few more years. Partisanship may be the only thing standing in its way.”

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The findings appear in the University of Chicago Legal Forum

Michael Heaney