Few votes were changed by the final televised debate between President Reagan and Walter F. Mondale, according to the latest Washington Post-ABC News poll.

The survey, taken Monday and Tuesday nights, showed the president heading into the final fortnight of the campaign with a 54-to-42 percent lead over his Democratic challenger. That is the same margin the Republicans held in the pre-debate Post-ABC poll taken Oct. 12-16.

The failure of Mondale to budge the numbers in his last face-to-face confrontation with Reagan highlighted the difficulty he faces in overcoming the lead Reagan has held since the campaign got under way.

Mondale pollster Peter D. Hart, confirming that he had similar figures, said the phenomenon that "amazes me is the remarkable consistency in Reagan's numbers over the last five months. Our challenge remains to convert some of those weak Reagan voters," he said, "and we have less time."

John Buckley, a spokesman for the Reagan campaign, said, "Our polling shows some movement toward Reagan since the Sunday debate and we believe the margin is larger than 12 points." Other officials said the lead in Reagan's polls was about 17 points.

A new Harris Survey released today shows Reagan leading 56-42 percent. Last week, the margin in the Harris Survey was nine points.

In a similar Post-ABC poll taken in the two days after their first debate Oct. 7, Mondale shaved 3 points from Reagan's advantage and cut another 3 points in the next week.

Mondale was judged the clear winner of that first debate, while Reagan had a 38-to-25 percent advantage over Mondale among those interviewed as the winner of Sunday's session in Kansas City. Among Reagan backers, 68 percent said the president won the second debate, 4 percent said Mondale did and the rest thought neither had won. Among Mondale supporters, 58 percent said the former vice president won, 4 percent said Reagan won and the rest saw neither as the winner.

The Post-ABC poll is based on call-back interviews with 1,081 registered voters who were surveyed in the pre-debate poll. More than seven out of 10 polled said they had watched some or all of the second debate.

Three out of 10 respondents said they thought Reagan was too old to serve a second term as president -- about the same percentage who had held that opinion before the debate.

Mondale's approval rating, which took a dramatic leap upward after the first debate in Louisville, showed a slight gain this time, going to 53 percent-44 percent favorable from 50 percent-44 percent. But Reagan's rating also improved slightly, to 61 percent-36 percent favorable from 60 percent-36 percent, and remained substantially higher than Mondale's.

Overall, what was most striking about the poll was the lack of movement -- an indication that the persistent Reagan lead may be solidifying in the final stage of the campaign.

The last three Post-ABC polls have shown Mondale drawing even with Reagan only in the East, but trailing by double-digit margins in the other three sections. The Democrat is splitting the vote with Reagan among those with incomes of less than $20,000 and those who consider themselves working-class, but is losing by wide margins among higher-income voters and those who consider themselves middle-class.

There is a sharp racial polarization in the vote, with Reagan getting six of every 10 white votes in the surveys and Mondale getting more than eight of every 10 black ones. The gender gap is there, but Mondale has not been able to gain enough women to offset Reagan's wide lead among men.

About 4 percent of those surveyed remain undecided.