FOR WANT of a nail, the shoe is lost. So begins an old refrain, which may be of consolation to former representative Toby Moffett of Connecticut as he contemplates his loss at last week's Democratic state convention.

Connecticut nominates its candidates for statewide office at party conventions, but a losing candidate who gets 20 percent of the votes at a July convention can challenge the winner in a September primary. Gov. William O'Neill, after five years in office, presiding over a state with a defense contract boom and one of the nation's lowest unemployment records, entered the convention with a solid majority. Still, Mr. Moffett hoped he could get the magic 20 percent: 270 votes out of 1,098. He got 250 instead.

This was not a remarkable result: an incumbent winning renomination in a prosperous state. But it was an especially vivid illustration of the old maxim that every vote counts. Mr. Moffett's supporters, brandishing walkie-talkies and resplendent in their red shirts, prowled the aisles looking for the 20 votes they needed to switch. They couldn't find them. The delegates had been elected in primaries in Connecticut's cities and towns on slates pledged to each of the candidates. Some of those results were close indeed. Most agonizing for Mr. Moffett must have been his defeat in Waterbury May 20. There 12,083 Democrats voted, and by a 43-vote margin gave Mr. O'Neill all of the city's 49 delegates.ju Mr. Moffett tried to get a court to overturnju the result -- that would have put him on the September ballot. But the court said he hadn't challenged the result within the time limit set by state law.

Forty-three votes! For an old political organizer and former Nader's Raider like Mr. Moffett, getting an extra 43 voters to the polls May 20 should have been easy. But he didn't do it, and so he will not get to use his skills as a statewide vote-getter -- he ran strong against Sen. Lowell Weicker in 1982 -- to get to the governor's office. You may not care much about who is governor of Connecticut or whether the Democrats there have a primary. But next time you're tempted to skip voting, just remember that 43 votes made perhaps the significant difference in determining who would be the governor of a state with 3 million people. Every vote does count.