Ron Edmonds, Associated Press
Homeland security adviser Fran Townsend speaks from the White House on Feb. 23, 2006. She is reportedly quitting post to pursue private-sector opportunities.

WASHINGTON — Presidential homeland security adviser Fran Townsend announced her resignation Monday, leaving an opening in a critical slot at a critical time.

Townsend, assistant to the president for homeland security and counterterrorism, plans to leave the post she has held since 2004 shortly after Jan. 1.

No replacement was named, but White House Press Secretary Dana Perino said, "We are hoping that there would be one relatively soon."

That would be good, said homeland security expert David Heyman at the Center for Strategic and International Studies, because of the increasing likelihood of terrorist attacks as the presidential election gets closer.

Heyman noted terrorist attacks that coincided with elections in Spain and Great Britain in recent years and Osama bin Laden's release of a videotape in the weeks before the 2004 presidential election.

"It is not at all inconceivable that we will need to be at a heightened level of concern for possible attacks," Heyman said. "You want somebody at the White House who is not just coming up to speed on how the infrastructure works and who is a trusted adviser to the president."

Heyman also noted that the 2008 election also will mean the first-ever transition of the homeland security and counterterrorism mechanisms put in place after the Sept. 11 attacks.

"We are transitioning the largest innovation in governance and national security for the first time since World War II," Heyman said.

In a statement, President Bush said Townsend "always provided wise counsel on how to best protect the American people from the threat of terrorism."

"We are safer today because of her leadership," Bush said.

Townsend held the top deputy post in the White House homeland security unit when Bush promoted her in 2004. Now 45, Townsend also has served as an assistant Coast Guard commandant, a Justice Department official and a federal prosecutor in Brooklyn.

Townsend submitted a three-page handwritten resignation letter dated "November 2007." "My time here and every day of my nearly 23 years of public service have been both a blessing and a privilege," she told Bush in the letter.