He stood by Charles Sumner when that great and dangerous attempt was made to banish him from public life in the year 1862.

It was a time when Charles Sumner, as he told me himself, could not visit the college where he was graduated, and be sure of a respectful reception, when a very important Republican paper, the most important and influential Republican paper in Massachusetts, declared that Charles Sumner could not address a popular audience in New York with personal safety; when, under the lead of the United States District Attorney, one of the most successful managers of a political meeting who ever existed in Massachusetts, an attempt was made to defeat a resolution of confidence in him, in the Republican State Convention (when the whole of the House of Representatives, or of the Caucus, or of the Convention, was on one side and Richard H.Dana was on the other, it was about an even chance which came out ahead), Thayer stood by Mr.Sumner in that memorable State Convention, and helped save his great career to the country and to liberty.
He was a devoted supporter of John A.Andrew.

Andrew had been Governor the traditional three years, and there were men eager to supplant him.

When Adin heard of a formidable meeting called for that purpose, he exclaimed--I remember very well the indignation with which he said it--"They shall not lay their hands on the Lord's Anointed." He sent a message to the meeting that he would fight their candidate in every school district in Massachusetts.

The scheme was abandoned.
He was largely responsible for the defeat of the scheme for substituting biennial for annual election, and biennial sessions of the Legislature for yearly sessions in Massachusetts, although it did not receive its deathblow at the hands of the people until after his death.
But his chief service, after all, was in keeping the government of Massachusetts clean and incorruptible, at the time of the great raid which was made upon the Republican Party in the years between 1871 and 1883.