To change horses in midstream refers to someone literally trying to move from one horse to another while crossing a stream. Over time, it has also come to mean to make major changes after something has already begun.
On January 12, 2000 the Worcester Telegram and Gazette newspaper in Massachusetts reported on the 6-month moratorium on cell tower applications in the town of Spencer. It reported the following:
I don’t think we should change horses in midstream,” said Mr. Hicks. Both Mr. Hicks and Mr. Cloutier argued that the full board should be involved in the process leading to any decision whether to keep the current law firm or hire another.
David Lawrence wrote a news story for the Lewiston Daily Sun newspaper entitled, “Convention Ignores New World Crisis” that was published on July 12, 1960. The story was about the national political conventions and the crisis going on in the world that could lead to war. He wrote in part:
As recently as 1956, the pressure of international issues was evident, and during the campaign the Suez crisis helped the Republicans because the country was in no meed to “change horses in midstream.”
The Arizona Republican reported in its story, “Oppose Change In Organization Of G.O.P. Committee” published on September 17, 1920:
In the belief that it would be the height of folly to change horses in midstream, Republican nominees for congress and state office have united in an effort to preserve the present efficient organization of the Republican state committee.
Now it may not be strictly a favourite expression of Republicans in the United States, but Republicans certainly appear to use the expression more often than Democrats.
To wit, a variation of the expression was popularized by, but did not originate with, Abraham Lincoln in a speech in 1864 when he discovered that the National Union League was supporting him for a second term as President.
Abraham Lincoln told the Republicans upon accepting his renomination that the honour had not come because he was the best man but because Republicans had come to the conclusion that “it is not best to swap horses while crossing the river.” He added further, “I am not so poor a horse that they might not make a botch of it in trying to swap.”
The expression “don’t swap horses while crossing the river” had been around earlier in the century and evolved into today’s “don’t change horses in midstream.”
Historically Speaking 