As a politician and as president, Lincoln was a profound student of the Constitution and constitutional history.
The income tax became law during the Special Session of Congress that was called July 4, 1861. This unapportioned income tax was unconstitutional. Also established was the Internal Revenue Bureau, later renamed the Internal Revenue Service (IRS) to enforce the then unconstitutional income tax.
https://en.wikipedia.org/wiki/Commissioner_of_Internal_Revenue
The list of commissioners of the IRS shows the first commissioner as George S. Boutwell, whose term ran from July 17, 1862 to March 4, 1863.
https://emergingcivilwar.com/2021/07/27/us-government-financing-of-the-civil-war/
Personal Income Tax The Revenue Act of 1861 was passed to increase import tariffs, property taxes, and for the first time, to levy a flat rate income tax of 3% on incomes above $800. Its drawback was that it lacked a comprehensive enforcement mechanism. Thaddeus Stevens, chairman of the House Committee of Ways and Means Committee, avowed, “This bill is a most unpleasant one. But we perceive no way in which we can avoid it and sustain the government. The rebels, who are now destroying or attempting to destroy this Government, have thrust upon the country many disagreeable things.”
In 1862, President Lincoln signed a law imposing a graduated income tax. The law levied a 3% tax on incomes between $600 and $10,000 and a 5% tax on higher incomes. The law was later amended in 1864; it levied a 5% tax on incomes between $600 and $5,000, a 7.5% tax on incomes in the $5,000-$10,000 range and a 10% tax on all higher incomes.
1861 saw the Revenue Act with an income tax. It was passed during the Special Session of Congress that began July 4, 1861. 1862 saw the first commissioner of the Internal Revenue Bureau take office to enforce the income tax.
H.R. 54, Inroduced July 16, 1861. Amended July 29, 1861. Became law as Act of August 5, 1861. 12 Stat. 292.
Income tax at Sections 49 and 50.
[Start of Act] https://memory.loc.gov/cgi-bin/ampage?collId=llsl&fileName=012/llsl012.db&recNum=323
[Sections 49 and 50] https://memory.loc.gov/cgi-bin/ampage?collId=llsl&fileName=012/llsl012.db&recNum=340
https://memory.loc.gov/cgi-bin/ampage?collId=llhb&fileName=037/llhb037.db&recNum=182
IN THE SENATE OF THE UNITED STATES.
JULY 27, 1861.
Ordered to be printed.
AMENDMENT
Proposed by Mr. SIMMONS to the "bill (H. R. 54) to provide
increased revenue from imports, to pay interest on the public
debt, and for other purposes," viz: insert the following additional
sections.
1 SEC. 8. And be it further enacted, That from and after the
2 first day of January next there shall be levied, collected, and
3 paid, upon the annual income of every person residing in the
4 United States, whether such income is derived from any kind
5 of property, or from any profession, trade, employment, or voca-
6 tion carried on in the United States or elsewhere, or from any
7 other source whatever, if such annual income exceeds the
8 sum of one thousand dollars, a tax of five per centum on the
9 amount of such excess of such income above one thousand
10 dollars: Provided, That upon such portion of said income as
11 shall be derived from interest upon treasury notes or other
12 securities of the United States, there shall be levied, col-
13 lected, and paid a tax of two and one-half per centum.
14 Upon the income, rents, or dividends, accruing upon any prop-
15 erty, securities,- or stocks owned in the United States by any
16 citizen of the United States residing abroad, there shall be
17 levied, collected, and paid, a tax of seven and one-half per
18 contum, excepting that portion of said income derived from
19 interest on treasury notes and other securities of the govern-
20 ment of the United States, which shall pay two and one-half
21 per centum. The tax herein provided shall be assesed upon
22 the annual income of the persons herein named for the year
23 next preceding the time for assessing said tax, to wit: the
24 year next preceding the first of January, eighteen hundred
25 and sixty-two; and the said taxes, when so assessed and
26 made public, shall become a lien on the property or other
27 sources of said income for the amount of the same with the
28 interest and other expenses of collection until paid.
1 SEC. 9. And be it further enacted, That it shall be the
2 duty of the President of the" United States, and he is hereby
3 authorized, by and with the advice and consent of the Senate,
4 to appoint one principal assessor and one principal collector
5 in each of the States and Territories of the United States, to
6 assess and collect the taxes imposed by the first section of
7 this act, with authority in each of said officers to appoint so
8 many assistants as the public service may require, to be
9 approved by the Secretary of the Treasury.
[...]
woodpusher:
"The income tax became law during the Special Session of Congress that was called July 4, 1861.
This unapportioned income tax was unconstitutional.
Also established was the Internal Revenue Bureau, later renamed the Internal Revenue Service (IRS) to enforce the then unconstitutional income tax." Lincoln's income tax was certainly not considered unconstitutional at the time, and one way we can understand this is to remember that the first proposed wartime income tax came from our Father of the Constitution, during Pres. Madison's War of 1812.
If our Father of the Constitution considered a wartime income tax entirely constitutional in 1812, then on what grounds would a Pres. Lincoln, or Congress, decide not to use the same tax to fund Civil War in 1861?
Mr. Madison's War of 1812 ended before Congress was forced to pass his proposed income tax, but sadly, that was not the case in 1861.
1814 Treaty of Ghent eliminated a need for a US wartime income tax proposed by Pres. Madison.
Note US Sec. of State, future Pres. John Quincy Adams, center:
