WASHINGTON — Hillary Rodham Clinton has decided to end her historic presidential campaign while leaving her options open to retain her delegates and promote her issue agenda, a campaign official says.

The former first lady told House Democrats during a private conference call today that she will express support for Barack Obama's candidacy and congratulate him for gathering the necessary delegates to be the party's nominee.

"Senator Clinton will be hosting an event in Washington, D.C., to thank her supporters and express her support for Senator Obama and party unity. This event will be held on Saturday to accommodate more of Senator Clinton's supporters who want to attend," her communications director Howard Wolfson said.

Also in the speech, Clinton will urge once-warring Democrats to focus on the general election and defeating Republican presidential candidate John McCain.

The announcement brought to a close an epic five-month nominating battle pitting the first serious female candidate against the most viable black contender ever.

Obama last night secured the 2,118 delegates to claim the Democratic nomination, but Clinton stopped short of acknowledging that milestone, saying she would.

An adviser said Clinton and her lieutenants had discussed various ways a presidential candidacy can end, including suspending the campaign to retain control of her convention delegates and sustain her visibility in an effort to promote her signature issue of healthcare.

The other options include freeing her delegates to back Obama and ending her candidacy unconditionally. The official stressed that neither Clinton nor her inner circle had decided specifically what course to take other than to recognize that the active state of her bid to become the nation's first female president had ended.

On the telephone call with impatient congressional supporters, Clinton was urged to draw a close to the contentious campaign, or at least express support for Obama. Her decision to acquiesce caught many in the campaign by surprise and left the campaign scrambling to finalize the logistics and specifics behind her campaign departure.