YOU DON'T often find practicing politicians following the example of Sidney Carton; the far, far better things they do tend to be things that advance their careers rather than end them. Which makes all the more odd the decisions of a couple of practical politicians this month to renounce their ambitions to run for governor and to step aside for another.

One example comes from Dixie. South Carolina Rep. Tom Hartnett (R-S.C.) wanted to run for governor, but faced competition from his fellow representative, Carroll Campbell (R-S.C.). Some national strategists hoped to get Mr. Hartnett to take on Sen. Ernest Hollings (D-S.C.), but even a phone call from President Reagan that got Mr. Hartnett off the tennis court failed to get him to turn his ambitions away from Columbia. But he did adjust his aim slightly. Instead of governor, Mr. Hartnett now says, he will run for lieutenant governor as part of a team with Mr. Campbell. That sort of renunciation is, let us say, unusual for a politician.

Another example comes from Illinois. Neil Hartigan, the current attorney general of the state, is a machine Democrat from Chicago with good government credentials. Since he was elected lieutenant governor in 1972, it has been known that the governor's office has interested him. But this past week, just after a poll showed him running less well against Republican Gov. James Thompson than Adlai Stevenson III, Mr. Hartigan abruptly withdrew in Mr. Stevenson's behalf. For another four years as attorney general he turned down a nomination and an office that might well have been his.

Are these acts of renunciation and political selflessness the harbingers of a trend? It is possible, of course, that George Bush will agree to seek another four years as vice president as the running mate of Jack Kemp, or that Edward Kennedy will leave the 1988 presidential race so that Joe Biden can have his chance. It is possible, too, that the sun will rise tomorrow in the west. But don't hold your breath. We wish Messrs. Hartnett and Hartigan all good fortune. But we suspect that most practicing politicians will, like Messrs. Campbell and Stevenson, prefer Charles Darnay's fate.