Boutwell, an abolitionist, is known primarily for his leadership in the formation of the Republican Party, and his championship of African American citizenship and suffrage rights during Reconstruction.

At the turn of the 20th century, he abandoned the Republican Party, opposed the acquisition of the Philippines, and in 1900 supported Democrat William Jennings Bryan for president.

[19] His elective successes, sometimes in the face of major Whig victories statewide, highlighted Boutwell's potential, and brought him into the Democratic Party's leadership circles.

[20] He sat on the judiciary and finance committees, where he gained a reputation for thorough research into legislation, and advocated positions favoring free trade, restraint of the money supply, and increased taxes for spending on education and other reforms.

Outrage over the extension of slavery into territories acquired in the Mexican–American War increased the popularity of the Free Soil Party, but they and the Democrats were unable to unite to unseat the Whigs who dominated state politics until 1850.

On the Democratic side, Boutwell and Nathaniel Prentice Banks agreed with Free Soilers Sumner and Wilson on a division of offices should the coalition win.

[30] Boutwell was criticized by Free Soilers for taking a hands-off approach to the contentious election of Sumner, neither supporting nor opposing him during the balloting in the state senate.

A "Maine law" temperance reform bill was also approved, but Boutwell was criticized by the Whigs for vetoing the first version of it and then signing the second, allegedly under pressure from Free Soilers.

[39] The triumph of slavery will perpetuate confusion and discord among the states, civil war in the territories ... After the convention, Boutwell took up the study of law in the office of Joel Giles, a patent lawyer from Groton.

[45] Boutwell attended the Peace Conference of 1861 in Washington, D.C., which attempted to prevent the impending Civil War, and served as a liaison between the federal government and Massachusetts Governor John Albion Andrew in April 1861.

[46] In the peace conference, he angrily rejected Southern proposals favoring the extension of slavery and its enforcement in northern states, arguing that "the Union is not worth preserving" if such measures are needed to do so.

[51] He oversaw the growth of the bureau to some 4,000 employees, the largest department in the government, and Secretary of the Treasury Salmon P. Chase praised him for having the "highest obtainable ability and integrity".

A dark-skinned man, dark-eyed, dark-haired, thin in the flank, vigilant, self-contained, quiet; giving you the impression that he would wake up quick and in strength.

A speech from him is premeditated logic of inwoven facts and figures, delivered in a magnetic current which flows to the nerves of every man in his audience, however great he may be, and which penetrates through and through.

"[56] On July 4, 1865, after the Civil War ended, Boutwell gave a speech that advocated African American suffrage, echoing Thomas Jefferson's principal view from the Declaration of Independence that "all men are created equal.

[58] Boutwell served on the Joint Committee on Reconstruction, which framed the Fourteenth Amendment, which gave African American freedmen citizenship and established the inviolability of the United States public debt.

[51] "Mr. Boutwell is the last survivor of the Puritans of a bygone age," the French reporter Georges Clemenceau wrote, "a man after the heart of John Bunyan, too much of a fanatic to command the attention of the Senate, but too honest and sincere for his opinions to be ignored by his party.

Unlike his colleagues, a hostile observer wrote, he brought to the cause "the advantage of a cultivated mind, an extensive reading and a scholarly acquaintance with all of history that could be mustered into such a service".

[62][63] He did not expect impeachment to pass and did not foresee the Senate convicting Johnson; rather he hoped for a statement from the House that the president had committed high crimes and misdemeanors, in effect a resolution of censure).

His speech was not particularly notable for its rhetoric, but defense lawyer William Evarts seized on Boutwell's strained analogy of casting Johnson into deep space to provoke significant laughter and applause.

Without the approval or knowledge of President Grant or other Cabinet members, Boutwell began to release gold from the Treasury and sell government bonds, in order to reduce the supply of paper currency.

[88] Gould successfully maneuvered an informant, Daniel Butterfield, into a post as assistant to Boutwell and began buying gold in earnest, sending the price up.

[94] In order to implement the banking syndicate, Boutwell had to temporarily raise the national debt more than half of one per cent, for which he was accused of technically violating the law.

[99] Boutwell did not forget the plight of African Americans in the South who were subject to violence perpetrated by white Southerners, particularly the Ku Klux Klan.

The following winter, Grant nominated Butler's ally William Simmons for the Collectorship of the Port of Boston, the most powerful federal patronage position in Massachusetts.

He also remained a strong supporter of federal protection for Black voters in the South, backing the 1875 Civil Rights Law, which banned discrimination by common carriers and in public accommodations.

These elections were accompanied by significant orchestrated violence aimed at preventing African Americans from voting, and resulted in the return of Democrats to power there.

[106] After Boutwell left the Senate, President Rutherford B. Hayes appointed him in 1877 to prepare an updated edition of the Revised Statutes of the United States.

[107] During the 1880s and 1890s Boutwell practiced international and patent law from offices in Boston and Washington, D.C. His business included working for the United States and other national governments as counsel to several bilateral diplomatic commissions.

[109] In the late 1890s, Boutwell became increasingly disenchanted with the imperialist foreign policy of President William McKinley and left the Republican Party after the annexation of the Philippines following the 1898 Spanish–American War.