Opposing the war

That would be the Mexican War of 1846-48, in this case.  Southern slaveowners wanted a war against Mexico to secure the Republic of Texas as a slave state in the United States.  When General Zachary Taylor claimed that his troops had been attacked by Mexican troops on American soil, that became the pretext for war.

The war also aroused intense opposition among Northerners who were becoming increasingly alarmed at the increasing strength of what they called the Slave Power (the pro-slavery Southern bloc in Congress). 

Whig Congressman Abraham Lincoln (the Republican Party didn't exist yet) was an opponent of that war.  In one of his most famous pre-Civil War speeches, Lincoln argued in a speech to Congress of 01/12/1848 that President James Polk had begun the war under false pretences and that the war itself was wrong.  In this particular part of the speech, Lincoln addresses the alleged initial Mexican aggression, the meaning of which depended on whether Mexico legitimately claimed jurisdiction over the territory between the Nueces and Rio Grande rivers.  He challenged the president's honesty on the reason for beginning the war:

I am now through the whole of the President's evidence; and it is a singular fact, that if any one should declare the president sent the army into the midst of a settlement of Mexican people, who had never submitted, by consent or by force, to the authority of Texas or of the United States, and that there, and thereby, the first blood of the war was shed, there is not one world in all the president has said, which would either admit or deny the declaration.  This strange omission, it does seem to me, could not have occurred but by design.  My way of living leads me to be about the courts of justice; and there, I have sometimes seen a good lawyer, sturggling for his client's neck, in a desperate case, employing every artifice to work round, befog, and cover up , with many words, some point arising in the case, which he dared not admit, and yet could not deny.  Party bias may help to make it appear so; but with all the allowance I can make for such bias, it still does appear, to me, that just such, and from just such necessity, is the President's struggle in this case. ...

... I more than suspect ... that he is deeply conscious of being in the wrong - that he feels the blood of this war, like the blood of Abel, is crying to Heaven against him.  That originally having some strong motive - what, I will not stop now to give my opinion concerning - to involve the two countries in a war, and trusting to escape scrutiny, by fixing the public gaze upon the exceeding brightness of military glory - that attractive rainbow, that rises in showers of blood - that serpent's eye, that charms to destroy - he plunged into it, and has swept, on and on, till, disappointed in his calculation of the ease with which Mexico might be subdued, he now finds himself, he knows not where.

Lincoln was also concerned to defend himself against suggestions from the Slave Power that his opposition to the war was unpatriotic:

When the war began, it was my opinion that all those who, because of knowing too little, or because of knowing too much, could not conscientiously approve the conduct of the President, in the beginning of it, should, nevertheless, as good citizens and patriots, remain silent on that point, at least till the war should be ended.  Some leading democrats, including Ex President Van Buren, have taken this same view, as I understand them; and I adhered to it, and acted upon it,until since I took my seat here; and I think I should still adhere to it, were it not that the President and his friends will not allow it to be so.  Besides the contunal effor tof the Presient to argue every silent vote given for supplies, into an endorsement of the justice and wisdom of his conduct ...

Part of why this speech is so famous is a part that came back to embarass Lincoln in a few years, the part in which he expounded on the right of revolution.  He, of course, took a dim view of Jefferson Davis' particular application of that theory.

Historian Arthur Schlesinger, Jr., in his new book War and the American Presidency (2004) describes the opposition to the Mexican War in this way:

The Mexican War was almost as unpopular [as the War of 1812].  There was fierce opposition to the declaration of war. "People of the United States!" creid the famous editor of the New York Tribune, Horace Greeley, "Your rulers are precipitating you into a fathomless abyss of crime and calamity! ... Awake and arrest the work of butchery ere it shall be too late to preserve your soulds from the guilt of wholesale slaughter!"

The Massachusetts legislature passed a resolution declaring that the war, "so hateful in its objects, so wanton, unjust and unconstitutional in its origin and character, must be regarded as a war against freedom, against justice, against the Union."  Thoreau wrote his plea for "The Duty of Civil Disobedience," and James Russel Lowell condemned the war in his long satiric poem Biglow Papers.  "The United Staes will conquer Mexico," said Ralph Waldo Emerson, "But it will be as the man swallows the arsenic, which brings him down in turn.  Mexico will poison us."  (Karl Marx, on the other hand, defended the war, asking sarcastically whether "it was such a misfortune that glorious California has been wrenched from the laxy Mexicans.")

In the midterm elections of 1846, the administration of James K. Polk lost thiry-five seats and control of the House of Representatives.  The new House passed a resolution [on 01/03/1848] declaring that the Mexican War had been "unnecessarily and unconstitutionally begun by the President of the United States."  Talk about giving aid and comfort to the enemy!

Henry David Thoreau, the New England Transcendalist philosopher, went to jail for his refusal to pay taxes in protest of the Mexican War.  It was that experience which inspired him to write his famous essay (as Schlesinger mentioned) "Civil Disobedience," originally published in 1849 as "Resistance to Civil Government."  Thoreau was not generous in that essay toward those who merely politely declared their opposition to the war:

Practically speaking, the opponents to a reform in Massachusetts are not a hundred thousand politicians at the South, but a hundred thousand merchants and farmers here, who are more interested in commerce and agriculture than they are in humanity, and are not prepared to do justice to the slave and to Mexico, cost what it may.  I quarrel not with far-off foes, but with those who, near at home, cooperate with, and do the bidding of those far away, andwithout whom the latter would be harmless.  We are accusomed to say, that the mass of men are unprepared; but improvement is slow, because the few are not materailly wiser or better than the many.  It is not so important that many should be as good as you, as that there be some absolute goodness somewhere; for that will leave the whole lump.  There are thousands who are in opinion opposed to slavery and to the war, who, esteeming themselves children of Washington and Franklin, sit down with their hand in their poickets, and say that they know not what to do, and do nothing; who even postpone the question of freedom to the question of free-trade, and quietly read the prices-current along witht the latest advices from Mexico, after dinner, and, it may be, fall asleep over them both.  What is the price-current of an honest man and patriot today?  They hesitate, and they regret, and sometimes they petition; but they do nothing in earnest and with effect.  They will wait, well disposed, for others to remedy the evil, that they may no logner have it to regret.  At most, they give only a cheap vote, and a feeble countenance and Godspeed, to the right, as it goes by them.  There are nine hundred and ninety-nine patrons of virtue to one virtuous man; but it is easier to deal with threal possessor of a thing than with the temporary guardian of it.

Lincoln's dim opinion of President Polk was well-founded.  In The Year of Decision: 1846, (1943) historian Bernard DeVoto described him as follows:

The conquest of a foreign nation was the biggest enterprise on which, up to then, the American people had ever embarked.  ...

Polk thought with admirable realism about tariffs, the treasury, and the routine of domestic policy.  He thought with astonising shrewdness about the necessary political maneuvers of government.  But he thought badly about war.  He was willing to make war on either England or Mexico, if he should have to in order to accomplish his purpose.  But he believed that if there should be a war it could be won easily, probably without fighting, and certainly without great effort or expense.  Deliberately carrying twin torches through a powder magazine from Marc 4, 1845, to May 13, 1846, he made no preparation for either war [with England, which didn't happen, or with Mexico, which did].  He had no underatnding of war, its needs, its patterns, or its results.  The truth is that he did not understand any results except immediately ones.  He did not know how to make war or how to lead a people who were making a war.

Andrew Jackson wasn't particularly a fan of Polk's presidential policies.  In Andrew Jackson and the Course of American Democracy 1833-1845 (1984), Old Hickory's biographer Robert Remini writes, "About the only action by Polk to win Jackson's complete approval was the Presient' inaugural address, particularly the statement that the American claim to Oregon [against Britain] was 'clear and unquestionable.'"

But on the Mexican War?  Jackson agreed with Karl Marx on that one.  He supported the war.