Washington ? For the first time in a generation, the number of women serving in Congress could drop as a result of this fall’s midterm elections, an odd twist on a political season many had dubbed “the year of the woman” as female candidates made runs in several high-profile races.

If large clusters of Democratic incumbents lose in November, as expected, many women who are ousted will be replaced by men. Women candidates tend to do better in Democratic years, and 2010 is shaping up as a successful year for Republicans.

Women now hold 90 seats in Congress: 69 are Democrats and 21 are Republicans. After the November election, Congress could end up with as many as 10 fewer women members, prognosticators now say, the first backslide in the uninterrupted march of women to Washington since 1978.

Prospects of a setback left advocates of women’s rights in disbelief, but determined to try to prevent it.

“That is not going to happen,” said Terry O’Neill, president of the National Organization for Women, which is working to elect candidates of both sexes who support women’s equality.

While political attention has focused this year on Sarah Palin’s handpicked candidates, known as “Mama Grizzlies,” and a record number of Republican women running for House seats, primary losses have thinned their ranks to a double-digit few.

In fact, just four women are among the GOP’s 46 “Young Guns,” as the party calls its frontline challengers who are considered future leaders.

Republican campaign officials expect more women candidates will join the group as they prove themselves with the fundraising and organization needed to mount serious campaigns. But “the lioness’s share of Republican candidates in these really promising seats are men,” said David Wasserman, a congressional analyst at the Cook Political Report.