Most Congressional votes follow strict party lines these days. But the resolution authorizing the president to strike at Syria for using nerve gas against its own people scrambles those polarizing propensities.
Both parties are divided, and the president can only prevail by fashioning the sort of bipartisan coalition he has tried -- and largely failed -- to build around other issues.
Since the early '70s, the GOP has portrayed itself as more willing and able than the Democrats to employ American power. That tradition of robust military response helped generate widespread Republican support for President Bush's decision to invade Iraq after 9/11.
That tradition endures today, primarily in what might be called the interventionist wing of the Republican Party. And in those circles, it overrides the fact that the president is now a Democrat, not a Republican.
Speaker John Boehner reflected that tradition in a statement endorsing Obama's decision on Syria: "Only the United States has the capacity and capability to stop (Syrian President) Assad or warn others around the world that this type of behavior will not be tolerated."
Some Republicans are drawing on another tradition, embodied in the old adage "politics stops at the water's edge." Not all issues should be viewed through a political prism. The national interest still matters.
Rep. Mike Rogers, the chairman of the House Intelligence Committee, invoked that principle when he asserted on CNN, "This is a national security issue. This isn't about Barack Obama versus the Congress. This isn't about Republicans versus Democrats." Added Sen. John McCain: "A rejection of this resolution would be catastrophic ... for the institution of the presidency and the credibility of the United States."
A third Republican faction is impelled to support the president because it believes the real target of his "shot across the bow" is not Syria but Iran, a country whose nuclear ambitions pose a much greater threat to American -- and Israeli -- interests than the Assad regime in Damascus.
"The connection between Syria and Iran is clear as a bell," Sen. Lindsey Graham of South Carolina said on PBS.
Those arguments are not working, however, with the isolationist wing of the Republican Party. Sen. Rand Paul rejects the essence of Boehner's position. "I don't see American interests involved on either side of the Syrian war," he said on NBC's "Meet the Press."
For others, the legacy of interventionism has taken its toll on Republican voters. "My constituents are war-weary," Sen. Saxby Chambliss of Georgia said on CBS' "Face the Nation." "They don't want to see us get involved in this."
This weariness is reinforced for some Republicans by an adamant hostility toward President Obama, even when he espouses a policy they might otherwise support. "There are plenty of conservatives who are not isolationists who don't trust Obama to conduct foreign policy," a GOP strategist told the Washington Examiner. "They're not comfortable risking American troops with Obama as commander-in-chief."
The Republican divisions are more visible, but Democrats have their own problems. The war-weariness Sen. Chambliss mentions is even more pronounced among liberals who opposed the Iraq war in the first place. Only 29 percent of Democrats favor air strikes, according to a new Pew poll and that fatigue is compounded by confusion over where the president's strategy could lead.
"(The) aftermath of a U.S. strike on targets in Syria is difficult to predict, with negative consequences that may be beyond our capability to control," warned Sen. Ed Markey of Massachusetts.
But even longstanding Democratic doves are more willing to use force when it comes to defending Israel and deterring Iran, and that's why the administration is vigorously pushing that angle. Secretary of State John Kerry even compared Syria's Assad to Adolf Hitler and Saddam Hussein, and the argument is working with lawmakers like Eliot Engel of New York.
"If we stand idly by," Engel said on NBC's "Today," "then every despot in the world thinks they can commit war crimes and no one will do anything."
Obama's best argument with Democrats, however, is self-interest. He will be president for more than three more years. Many issues Democrats care about are coming up on Capitol Hill: immigration, tax reform, entitlement spending. These legislators are stronger if he is stronger, and losing the Syrian vote would diminish the president's leverage in all those battles.
So here's the key to a winning coalition: liberal Democrats who want to strengthen the president and conservative Republicans who want to strengthen the presidency. Our bet is they will narrowly prevail.
Steve and Cokie Roberts can be contacted by email at email@example.com.