Some don't like war, any war. Others don't think the United States should get involved in this particular war. And some simply don't like anything President Barack Obama proposes.
Those dynamics amount to a tough challenge for Obama to win support from Congress, particularly the Republican-led House of Representatives, for his proposed attack on Syria in response to what he calls the regime's use of chemical weapons on its own people.
Obama turned to Congress after Britain's Parliament rejected taking part in any military attack on Syria, depriving the president of a normally reliable ally.
With U.N. action undermined by opposition from permanent Security Council member Russia, Obama decided to seek political cover from Congress even though he insists he has the power to act without its authorization.
Opposition comes from across the political spectrum, with liberal Democrats and libertarian tea party Republicans in rare unity against what they call an unnecessary U.S. foreign intervention.
Closer to the center, more moderate legislators from both parties say they need more answers to questions about what the mission would be and why what is happening in Syria threatens U.S. national security.
Republican Rep. Jim Risch, an Idaho conservative, summed up questions by legislators asked to decide the matter.
"The reluctance is a whole list of things, not the least of which is once you open this can, what's going to come out?" he said, adding: "I still keep hearing that the objective is, 'well we have to do something.' That's not good enough for me at this point."
Obama got a huge boost on Tuesday when the top two Republicans in the House -- Speaker John Boehner of Ohio and Majority Leader Eric Cantor of Virginia -- fully backed him.
"This is something that the United States as a country needs to do," Boehner told reporters, adding he would support the resolution authorizing military strikes on Syria and that his House colleagues "should support this call for action."
Cantor issued a statement that said America "has a compelling national security interest to prevent and respond to the use of weapons of mass destruction, especially by a terrorist state such as Syria, and to prevent further instability in a region of vital interest to the United States."
"Understanding that there are differing opinions on both sides of the aisle, it is up to President Obama to make the case to Congress and to the American people that this is the right course of action, and I hope he is successful in that endeavor," Cantor's statement said.
Despite the backing from Boehner and Cantor, it remained unclear if Obama can muster enough House Republicans to join an equally uncertain number of House Democrats to back authorization of military strikes.
In a sign of the split among Republicans, the No. 3 GOP leader in the House -- Rep. Kevin McCarthy of California -- signaled unease with supporting a resolution authorizing military action against Syria.
"Absent a clear sense of what we must do, and what the mission is, it is difficult to formulate an appropriate and effective resolution authorizing the president to use military force," said a statement by McCarthy's spokesman, Mike Long.
Tea party conservatives, who comprise a sizable minority of the House GOP caucus, have defied Boehner repeatedly in recent years. On the Democratic side, the liberal-progressive wing generally votes anti-war.
"The strike is not going to accomplish anything useful." argued Democratic Rep. Alan Grayson of Florida on CNN, adding: "We are not the world's policemen. We're not the world's judge, jury, or executioner. This is not our responsibility."
House Democratic leader Nancy Pelosi of California conceded to reporters on Tuesday that "more work needs to be done."
"In my district, I don't think people are convinced that military action is necessary, but it's important for them to know that the weapons of mass destruction use has taken us to a different place," Pelosi said after meeting with Obama.
In a letter to Democratic members on Tuesday, Pelosi urged them to share with their constituents the details of what U.S. officials say was a major chemical weapons attack by Syrian President Bashar al-Assad's regime on Damascus suburbs.
While supporting Obama's call for action, Pelosi's letter also promised her caucus a voice in the debate, saying that "the shape and content of the final resolution will depend on what members can support."
In the Senate, Majority Leader Harry Reid of Nevada is confident the Obama resolution will pass, even if it requires 60 votes to overcome any filibuster, said a Democratic source familiar with Reid's thinking.
Democratic Sen. Robert Menendez of New Jersey, who chairs the Foreign Relations panel, said in opening remarks at a hearing on Tuesday that he supported military strikes on Syria.
"This is not a declaration of war, but a declaration of our values to the world," Menendez said.
To Darrell West, the vice president and director of governance studies at the Brookings Institution, support from the House is the biggest challenge for Obama.