(CNN) -- President Barack Obama ends four days of travel Wednesday with a speech to the National Urban League that will seek to energize the African-American base for his re-election bid in November.

A new report by the group's policy institute warns that low turnout by African-American voters, who overwhelmingly supported Obama in 2008, could hurt his chances this time in the battleground states of Virginia, North Carolina and Ohio.

"African-American voters tipped the outcome of the 2008 presidential election in several key states, and are poised to do so again in 2012," said the report titled "The Hidden Swing Voters: Impact of African-Americans in 2012" by Madura Wijewardena and Valerie Wilson.

"How this will manifest will depend on many things, but one important factor will be whether the extraordinary growth in turnout by African-American voters in 2008 will be replicated in 2012," the report continued. "The 2008 voter turnout rate was driven by historic factors that may not necessarily apply in 2012."

The "historic factors" reference was to Obama being the nation's first African-American nominee of a major party, with voters having the opportunity in 2008 to make him the nation's first African-American president.

This time, an economy struggling to recover from a recession that hit African-Americans particularly hard has raised questions about whether Obama supporters will have the same fervor as they did four years ago.

A recent Gallup poll showed the president with overwhelming support among registered African-American voters, with backing of 89%, compared with 5% for certain Republican nominee Mitt Romney. In 2008, Obama won 95% of the African-American vote, with 4% for GOP candidate John McCain.

The turnout by African-American voters made up 13% of the total vote in 2008, compared with 11% in 2004, according to CNN exit polls.

A decline in African-American voter turnout to the 2004 level of 60% from the 2008 level of 64.7% would cause Obama to lose in North Carolina and possibly lose in Ohio and Virginia, the National Urban League Policy Institute report said.

Obama won all three states in 2008, and most scenarios for Obama's re-election depend on him winning at least two of them this time. Ohio has 18 electoral votes, while North Carolina has 15 and Virginia has 13.

The president's speech concludes a four-day, six-state swing that started earlier than planned Sunday so that he could visit Aurora, Colorado, after last week's mass shooting at a movie theater.

Obama also held campaign events in California, Oregon and Washington; spoke to the Veterans of Foreign Wars convention in Nevada; and headed to New Orleans on Wednesday for more campaign events and the speech to the National Urban League, which started as a civil rights organization and now promotes economic empowerment in the nation's cities.

A fierce start to the election campaign, with the candidates and their supporting super-PACs launching bitter attacks, has made ensuring enthusiastic backing from traditional support bases a key to victory in November.

CNN's Tom Cohen and Paul Steinhauser contributed to this report.