"In the midst of the Confederate v United States trials, congress was mixed up in its own bloody battle. After the easy bipartisan passing of the 14th amendment (on the issue of succession) the republicans were ready to take their next big step. A 15th Amendment, one that would certainly not have the same level of bipartisan support. The 15th Amendment was to be on the issue of freedmen citizenship and more than that, freedmen suffrage. A law had been passed and signed in March that already addressed black citizenship (The 1866 Civil Rights Act) but it was seen as unenforceable and something larger would need to be done, something like an Amendment to the United State Constitution. While citizenship wasn't really that hard of a sell to many moderates in the republican caucus. Suffrage certainly was. The first seeds for this amendment had been planted from the speeches of the late President Lincoln, and had been evolving over the course of 1865 and 1866. By June of 1866 two versions of the 15th amendment had already been denied. In July a third version was presented. One that was radical, but not absurd. The most radical part was Section 4. The suffrage clause.
With the full backing of the Grant Administration the 15th Amendment was presented to the US House of Representatives on December 9th 1866. The Amendment easily passed through the republican owned House committees with little issue. Speaker Colfax was terrified that the suffrage clause would get this Amendment shot down. On July 5th, the House voted 145-71 to pass the 15th amendment by two votes. Next up was the senate. An objectively harder fight. Multiple more moderating amendments were proposed, including only granting voting rights to black veterans or removing the suffrage clause all together. These were shut down, albeit barely. Mostly due to the work of the Grant Administration, offering jobs and favors in exchange for votes. Thus on July 28th the Senate was going to vote on an un-altered 15th Amendment. With all eyes on the senate floor, the vote was 43-21 by exactly one vote the 15th Amendment was passed through congress. Two days later the Secretary of State Frederick Seward presented this Amendment to the States
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside. No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any state deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
Representatives shall be apportioned among the several states according to their respective numbers, counting the whole number of persons in each state, excluding Indians not taxed. But when the right to vote at any election for the choice of electors for President and Vice President of the United States, Representatives in Congress, the executive and judicial officers of a state, or the members of the legislature thereof, is denied to any of the male inhabitants of such state, being twenty-one years of age, and citizens of the United States, or in any way abridged, except for participation in rebellion, or other crime, the basis of representation therein shall be reduced in the proportion which the number of such male citizens shall bear to the whole number of male citizens twenty-one years of age in such state.
No person shall be a Senator or Representative in Congress, or elector of President and Vice President, or hold any office, civil or military, under the United States, or under any state, who, having previously taken an oath, as a member of Congress, or as an officer of the United States, or as a member of any state legislature, or as an executive or judicial officer of any state, to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof. But Congress may, by a vote of two-thirds of each House, remove such disability.
The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any state on account of race, color, or previous condition of servitude. 
The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned. But neither the United States nor any state shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.
The Congress shall have power to enforce, by appropriate legislation, the provisions of this article.
(The Grant Administration would make it that any former state that had left the union, and wanted to return would have to ratify this amendment first)
The trick up congresses' sleeve that would get this amendment ratified was the fact that no former Confederate State would get the chance to vote and deny it. Only 25 states would get that chance. (19 needed for ratification. 3/4) Thus while that increased the chances of its ratification, it wasn't assured. Already California and Oregon were hostile to it. The border states were livid and even states like Ohio and New Jersey were tossups. Though the Grant Administration did all it could to leverage favors all they could really do was wait and see.
The States that ratified, in order
Connecticut, August 9th 1866
Vermont, August 12th 1866
Illinois, August 18th 1866
New Jersey, August 19th 1866
New York, August 28th 1866
Massachusetts, September 10th 1866
New Hampshire, September 19th 1866
Michigan, September 20th 1866
Rhode Island, September 25th
Maine, September 27th 1866
Indiana, September 29th 1866
West Virginia, October 1st 1866
Pennsylvania, October 5th 1866
Kansas, October 8th 1866
Nebraska, October 17th 1866
Minnesota, October 18th 1866
Wisconsin, October 22nd 1866
Ohio, November 19th 1866
Nevada, November 26th 1866
States that rejected the Amendment, in order
Kentucky, September 9th 1866
Delaware, September 15th 1866
Missouri, October 8th 1866
Maryland, October 29th 1866
California, November 23rd 1866
Oregon December 10th 1866
The 15th Amendment had mixed reactions in the North. Some hailed it as true progress towards a more perfect union and others thought the federal government had gone too far. In the border states there was absolute fury and the amendment was practically ignored. Considering they weren't under occupation there was little the Grant administration could do. Moderates also denounced the Amendment as overreach but remained cautiously optimistic that it would be the most radical thing the Grant Administration supported."
-From an Overview of the Amendments to the Constitution
by Kid's Learning, published 2012
"1866 for the Grant Administration was a year that was meant to set the tone of the reconstruction to come. The Union v Confederate cases, 15th Amendment and increased funding of the Freedmen's Bureau. In March, President Grant resurrected the pocket vetoed Wade-Davis Bill, increasing the Oath of loyalty threshold to re-enter the union to 15 percent of a state's white population and not 10. President Grant also supported the breaking up of big plantations and re-distributing the land to former slaves. (Mostly through homesteading) He also worked with the Freedmen's Bureau to ensure that financial compensation was given to former slaves. Both of these measures made sure that freedmen wouldn't be forced to return to the cotton fields to make a living.
Congress also passed a series of "Reconstruction Acts" that the president quickly signed. These acts strengthened the military districts powers and deployed an extra 35,000 Soldiers to assist in the occupation. They also returned Washington D.C to its pre-war territory (re-taking the seized land that Virginia took during the war). The Reconstruction Acts were responses to increasing violence all over the south, rampant lynchings, lootings and burnings had been occuring left and right especially after Jefferson Davis' execution. President Grant also created the Department of Equity , headed by Attorney General Benjamin Wade, to assist in the prosecution of former Confederate officials in the Union v Confederate cases but also to assist in the prosecution of the southern White Leagues that popped up with more fervor post the ratification of the 15th Amendment. In May of 1867 Congress gave control over the Freedmen's Bureau to the DoE as well. In the south, the Attorney General and the DoE became the "villain of reconstruction" as Benjamin Wade essentially became the face of the occupation.
In late 1867 President Grant signed the Enforcement Act. Which essentially gave teeth to the 15th Amendment and its enforcement. Especially over Section Three which mandated
"No person shall be a Senator or Representative in Congress, or elector of President and Vice President, or hold any office, civil or military, under the United States, or under any state, who, having previously taken an oath, as a member of Congress, or as an officer of the United States, or as a member of any state legislature, or as an executive or judicial officer of any state, to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof. But Congress may, by a vote of two-thirds of each House, remove such disability."
President Grant wanted to ensure that no former Confederate soldier could hold a position on a Military Districts advisory board or be allowed as a delegate in a southern states' new constitutional convention. Said conventions were going to be hard to begin in their own rights. In order to hold any such convention under the Enforcement Act of 1867, 15 percent of the white population of the territory would have to swear an Oath of Loyalty, then the Military Governor and the Advisory board of the larger Military District would have to approve of the convention, and then the Department of Equity would have to approve. By November of 1868 only Tennessee had gotten approval from the DoE to hold a Constitutional Convention, which it did in late November. By the Election of 1868 no southern states had been re-admitted to the union.
-From Reconstruction: A Complete History
by Doris Goodman, published 1999
: Obviously this is the major change from OTL 14th Amendment.
: This is the DoJ from our TL.