Thomas Dewey was basically the poster child for a "tough on crime" prosecutor turned politician. He was an unemotional, egotistical guy who someone one described as "the only man who could strut sitting down" (source).
A powerful attorney in 1930s New York City, Dewey used his position to make a successful run at becoming the state's governor. He then began a lifelong pursuit of a moderate and progressive Republican Party, becoming deeply embroiled in political battles with both FDR and Truman. He used his position as governor as a stepping stone to number of presidential campaigns, but he failed to get the nomination in 1940 and lost to the unstoppable FDR in 1944.
In his presidential campaigns, his progressive Republicanism clashed with Robert A. Taft, the face of the Old Guard Republicans. The two men hated each other so completely that their divisive campaigns were a major factor in the national elections. The two men so split the party that it was virtually impossible for either to get the support of the party as a whole. Dewey prevailed and got the nomination.
In the 1948 Presidential election, the smart money was on Dewey. In every publisher's worst-case scenario, The Chicago Tribune, forced to go to press a few hours early because of an impending printers' strike, printed 150,000 copies of their paper with the headline: "Dewey Defeats Truman." For many people, that's what they remember most about Dewey: the iconic photo of a smiling, victorious Truman waving that paper in front of reporters.
You can see why Dewey decided not to run in 1952.
Instead, he focused his energies on convincing Eisenhower to run. In Eisenhower he saw a leader who'd unify the party and push his progressive agenda. He was right on both counts. Though Eisenhower narrowly defeated Taft in the primaries, his cross-party appeal led to victories over the Democrats in 1952 and 1956.
Dewey was also a great supporter of Nixon, whom he believed had a bright future in the progressive Republican Party of Dewey's dreams. If Nixon was the author of the Checkers Speech, then Thomas E. Dewey was its architect. He saw in Nixon a load of potential, and believed that he should remain on the ticket as long as popular opinion wasn't against it. It was his idea that Nixon should urge folks to write in to the Republican National Committee, an idea which resulted in an outpouring of supportive mail.
Wonder if he also volunteered to help them sort that mail.