Ohio, Illinois, Missouri, Nevada, and New Mexico have been the five most predictable states in Presidential elections in American history.
Ohio has participated in 54 of the 58 Presidential elections in American history since 1804.
It has often been said that Ohio is the “crucial” state in the quadrennial election process, and that is so true.
No state has had the impact of Ohio, and particularly, due to the fact that Ohio has participated in more elections than all states except the original 13 states, plus Vermont, Kentucky and Tennessee, and none of those have been as “predictable” in backing the winners of the election.
Altogether, Ohio has been “correct” in backing the winner all but 9 times, a total of 45 out of 54 times, or 83.3 percent of the time.
The exceptions are the following chronologically:
1824–Henry Clay over John Quincy Adams
1836–William Henry Harrison over Martin Van Buren
1844–Henry Clay over James K. Polk
1848–Lewis Cass over Zachary Taylor
1856–John C. Fremont over James Buchanan
1884–James G. Blaine over Grover Cleveland
1892–Benjamin Harrison over Grover Cleveland
1944–Thomas E. Dewey over Franklin D. Roosevelt
1960–Richard Nixon over John F. Kennedy
Illinois is the second most predictable states, having voted since 1820 for the winner all but 9 times in 50 elections, for a percentage of 82 percent.
The exceptions chronologically are:
1824–Andrew Jackson over John Quincy Adams
1840–Martin Van Buren over William Henry Harrison
1848–Lewis Cass over Zachary Taylor
1884–James G. Blaine over Grover Cleveland
1916–Charles Evans Hughes over Woodrow Wilson
1976–Gerald Ford over Jimmy Carter
2000–Al Gore over George W. Bush
2004–John Kerry over George W. Bush
2016–Hillary Clinton over Donald Trump
Note that Illinois voted for the popular vote winner in 1824, 2000 and 2016.
Missouri is the third most “predictable” state, coming into the Union in time for the Presidential election of 1820, so having voted in a total of 50 of the 58 national elections, and being with the winner 37 out of 50 times, or about 74 percent of the time.
The exceptions chronologically are as follows:
1824–Henry Clay over John Quincy Adams
1840– Martin Van Buren over William Henry Harrison
1848–Lewis Cass over Zachary Taylor
1860–Stephen Douglas over Abraham Lincoln
1872–Horace Greeley over Ulysses S. Grant
1876–Samuel Tilden over Rutherford B. Hayes
1880–Winfield Scott Hancock over James A. Garfield
1888–Grover Cleveland over Benjamin Harrison
1896–William Jennings Bryan over William McKinley
1900–William Jennings Bryan over William McKinley
1956–Adlai Stevenson over Dwight D. Eisenhower
2008–John McCain over Barack Obama
2012–Mitt Romney over Barack Obama
Note that Missouri voted with the popular vote winner in 1888.
Two other states also have a high consistency rate of accuracy voting for the winner of Presidential elections, but have fewer times of participation in Presidential elections.
Nevada voted for the winner 31 out of 39 times since 1864, 79.5 percent of the time, with the exceptions chronologically as follows:
1880–Winfield Scott Hancock over James A. Garfield
1884–James G. Blaine over Grover Cleveland
1892–James B. Weaver over Grover Cleveland
1896–William Jennings Bryan over William McKinley
1900–William Jennings Bryan over William McKinley
1908–William Jennings Bryan over William Howard Taft
1976–Gerald Ford over Jimmy Carter
2016–Hillary Clinton over Donald Trump
Note that Nevada voted with the popular vote winner in 2016.
Finally, New Mexico, in the Union since 1912, and therefore participating in 27 elections for President, has voted with the winner all but three times, 88.8 percent of the time, the exceptions being:
1976–Gerald Ford over Jimmy Carter
2000-Al Gore over George W. Bush
2016–Hillary Clinton over Donald Trump
Note that Al Gore and Hillary Clinton both won the popular vote, but lost the Electoral College to their opponents.