A record turnout of almost 6,000 Arlington Democrats voting in a three-day caucus chose Erik Gutshall for County Board on Saturday and opted to endorse School Board candidate Monique O’Grady over incumbent James Lander.

Gutshall defeated three other candidates, taking 3,209 votes, or 55 percent, in the second round of the instant-runoff vote, coming in ahead of Kim Klingler, who got 1,416 votes, or 24 percent, and Vivek Patil, who won 1,189 votes, or 20.5 percent. Peter Fallon came in fourth in the first round with 945 votes, so he did not make it to the final round.

O’Grady, a first-time candidate, beat veteran School Board member Lander by 1,100 votes, or a margin of 19 percentage points, after a late-campaign-season gaffe by Lander, who criticized domestic violence victim Yeardley Love in a local radio interview last month. Maura McMahon came in third.

“I hope I won because of the values I put on the table,” O’Grady said, citing her support for classroom instruction while the system struggles with crowding and the importance of diversity and of listening to teachers and students.

In the County Board race, ­Gutshall cited the endorsements he received this year and last year from many current and former elected and appointed leaders in the local Democratic Party, as well as his message of focusing on the future.

“I think of lot of people are ready to turn the page in Arlington,” said the 47-year-old small-business owner, who lost a hard-fought primary last year against incumbent Libby Garvey. ­Gutshall was the clear leader in fundraising this time, although he faced two others who had also previously run for office — Fallon was on his fourth try and Klingler on her second try for the County Board — and energetic newcomer Patil.

Gutshall is aiming for the seat being vacated by Jay Fisette (D), who is stepping down from the board after nearly 20 years. Fisette endorsed Gutshall within a day of his announcement.

The winner of the caucus is the presumptive victor in the general election because of the heavily Democratic voter base in Arlington. No Republican has yet announced a candidacy for the County Board. Perennial candidate Audrey Clement is in the race as an independent.

The caucus format was criticized earlier this spring by the Arlington Young Democrats, who wanted a primary so that there would be absentee voting, among other reasons. The party added a third day to the caucus, which was held Tuesday and Thursday nights, and all day Saturday. The unofficial turnout of 5,972 was the highest since 1993, when 4,951 caucusgoers voted.