He was the only Democrat to serve as speaker during the Progressive Era when Republicans dominated the House, Senate, and presidency.

Clark helped Wilson pass much of his progressive agenda but opposed U.S. entry into World War I.

In the 1918 midterm elections, Democrats lost their control of the House of Representatives, ending Clark's tenure as Speaker.

After a surprise loss in 1894 to William M. Treloar, he regained the seat in 1896, and remained in the House until his death, the day before he was to leave office.

Clark ran for House Minority Leader in 1903 but was defeated by John Sharp Williams of Mississippi.

"[7] The Conservative Party of Canada, which opposed the treaty, won the Canadian election in large part because of Clark's speech.

In 1912, Clark was the frontrunner for the Democratic presidential nomination, coming into the convention with a majority of delegates pledged to him, but he failed to receive the necessary two-thirds of the vote on the first several ballots.

Clark's speakership was notable for his skill from 1910 to 1914 in maintaining party unity to block William Howard Taft's legislation and then pass Wilson's.

Clark split the party in 1917 and 1918, when he opposed Wilson's decision to bring the United States into World War I.

In addition, Clark opposed the Federal Reserve Act, which concentrated financial power in the hands of eastern banks (mostly centered in New York City).