Rep. Brian Bilbray (R) led presumptive challenger Nick Leibham (D) by 18 points, according a poll of 50th district voters conducted recently for the Democrat’s campaign.

According to a survey of 400 likely general election voters by the Democratic firm Greenberg Quinlan Rosner Research, Bilbray topped Leibham, an attorney, 52 percent to 34 percent, with 13 percent undecided.

But Leibham’s campaign was encouraged by additional findings in the poll suggesting Bilbray could be vulnerable come November in the Republican-leaning San Diego area district.

“Nick Leibham has the ability to make this race competitive and defeat Brian Bilbray,” said a memo prepared by GQRR pollsters Ben Tulchin and Ron Eckstein.

The poll was conducted April 24-27, and had an error margin of 4.9 points.

Its other findings:

• Democrats held a 43 percent to 39 percent generic Congressional ballot lead over the Republicans, with 17 percent undecided.

• Voters who said they would definitely or probably vote to re-elect Bilbray trumped by a 42 percent to 39 percent margin those who said they would definitely or probably vote for someone else, with 20 percent undecided.

Bilbray narrowly won the 50th district in a hard-fought June 2006 special election that was closely contested by the Democratic Congressional Campaign Committee and the National Republican Congressional Committee.

Bilbray subsequently won an easy race for re-election in the November 2006 general election.

— David M. Drucker