The 1948 presidential election was the one where he made his suprise victory. Throughout the spring of '48 he was widely seen as being unable to win the election. There was a faction in the Democratic Party that even tried to swing the nomination to General Eisenhower. Given this backdrop, we have a sitting President that was unpopular enough to have to struggle to gain his own party's nomination.

Given his general unpopularity at the time and his desire to win the election, he conducted his famous "whistle-stop" tours across the country to gain popular support. Even though he gained more and more popular support, the establishment media kind of ignored the shift in momentum. Some of this was due to errors in polling techniques that showed his opponent, Thomas Dewey, was polling much more favorably. They even stopped polling in September. Most of the error in the polling data was related to a skewed sample, the polls were conducted via telephone and were highly skewed towards Dewey because most Truman supporters didn't even have a phone yet.

So certain was the press of Dewey's lock on the election that they even printed their Front Page articles proclaiming the election results "Dewey Defeats Truman" the night before. (Chicago Tribune).

Final election results were Truman 303, Dewey 189 and Thurmond 39 electoral votes. Truman won the popular vote by over 2.2 million votes. It turned out that it wasn't really that close a vote at all. Thurmond has split the democratic party but even with those votes lost Truman still had outpaced Dewey.

For the second part, about the opposition to his plan there was a recent NPR piece that covered this a bit.

http://www.npr.org/2017/07/09/536263099/political-debates-over-health-care-go-back-decades

Most of the AMA's reaction to Truman's plan centered on Socialist/Communist fears. Even when you get past those issues you find that the Republican response was that the government shouldn't be involved in "hand-outs". Some of the quotes in that piece are very explicit to their concerns.