The Speaker of the House of Representatives is second in line for the Presidency after the Vice President under the Presidential Succession Act of 1947, the third such law.
The first such law, from 1792-1886, put the Speaker third in line for the Presidency, with the Vice President and the President Pro Tempore of the US Senate ahead of him, later reversed in 1947.
The second law, from 1886-1947, did not include the Speaker in the line of succession, but rather the Cabinet officers after the Vice President.
In our history, only one Speaker of the House became President, James K. Polk of Tennessee, from 1845-1849, and he proved to be one of the more significant Presidents, adding more real estate to America than anyone other than Thomas Jefferson. This was accomplished by treaty with Great Britain over the Pacific Northwest in 1846, and by war with Mexico from 1846-1848, which added the Southwestern United States to the Union.
But seven other Speakers sought the Presidency, including the following:
Henry Clay of Kentucky sought the Presidency in 1824, 1832, and 1844, and is regarded as the greatest single legislator in the history of both houses of Congress. In 1844, we had the only Presidential election where the two opponents had both been Speaker of the House, Clay and Polk! Clay lost his three elections to John Quincy Adams, Andrew Jackson, and Polk.
John Bell of Tennessee was the Constitutional Union Party nominee for President in 1860 on the eve of the Civil War, and lost to Abraham Lincoln.
James G. Blaine of Maine was the Republican nominee for President in 1884 and lost the election to Grover Cleveland, and was also Secretary of State under three Presidents–James A. Garfield, Chester Alan Arthur, and a full term under Benjamin Harrison.
Thomas Reed of Maine lost the nomination of the Republican Party in 1896 to future President William McKinley.
Champ Clark of Missouri lost the nomination of the Democratic Party in 1912 to future President Woodrow Wilson.
John Nance Garner of Texas, after being Vice President under Franklin D. Roosevelt for two terms from 1933-1941, lost the nomination of the Democratic Party to his boss, President Franklin D. Roosevelt in 1940
Newt Gingrich of Georgia lost the Republican nomination for President to eventual nominee Mitt Romney in 2012.
So four Speakers were nominated for President, with only Polk winning; and four other Speakers lost the nomination when they sought the Presidency.
Now we may have a ninth such Speaker seeking the Presidency, Republican Speaker Paul Ryan of Wisconsin, whose name is being promoted, despite Ryan’s denial of any interest in running for President.