WASHINGTON (Reuters) - President Barack Obama on Tuesday will press Congress to approve $6.18 billion in emergency funding to help fight the Ebola outbreak in West Africa and prepare U.S. hospitals to handle future cases.

Most of the request is aimed at the immediate response to the disease at home and abroad. But the package also includes $1.5 billion in contingency funds - money that could become a target if lawmakers decide to trim the bill.

"That is the part of the package that is most at risk," said Sam Worthington, president of InterAction, an alliance of U.S. non-governmental aid groups.

While lawmakers recognize that the United States had to take action to arrest the deadly disease, some are wary of giving the administration leeway in investing money in public health systems in West Africa.

"I think there is less understanding of the need to stay in it for the long run and to build the capacity of countries to ensure this doesn't happen in the future," Worthington said in an interview.

The worst Ebola outbreak on record has killed at least 5,987 people since March, mainly in Guinea, Liberia and Sierra Leone.

The Obama administration came under fire in September after a series of missteps with a man who traveled to Dallas from Liberia and later died of Ebola. Two nurses contracted the disease while caring for the man.

Screening and treatment procedures have since been tightened. There are no current U.S. cases, and stories about the outbreak have faded from headlines.

"My hope is that we're not getting Ebola fatigue setting in," said Bruce Johnson, president of SIM USA, a Christian missionary group helps treat Ebola patients in Liberia. "There continues to be a huge need for this funding."

On Tuesday, Obama will tour and speak at the National Institutes of Health in Bethesda, Maryland, where a team of researchers last week published promising results from the first phase of a research trial for an Ebola vaccine.

The vaccine could go into testing in West Africa in the next few weeks - a sign of progress Obama will praise in his remarks, a senior administration official said in an interview.

The contingency funds requested from Congress could be used to help manufacture and distribute a successful vaccine, the official said.

"They're having questions about the contingency fund, but we think as we answer those questions, people understand why it's necessary," the official said.

(Reporting by Roberta Rampton; Editing by Lisa Shumaker)