Biden, the Democratic vice presidential candidate and an Irish Catholic from a Pennsylvania working-class background, could also win over blue-collar workers who backed Obama's party rival for the White House, Hillary Clinton.
"If we just focus on the religious element, adding Biden to the Democratic ticket was an extremely smart move," said Michael Lindsay of Rice University in Houston. Obama made Biden his running mate on Saturday.
"Biden has ties in a very important swing state, Pennsylvania, where Catholics are the biggest constituency whose votes are up for grabs," Lindsay told Reuters.
US presidential elections are often decided by swing states that move from party to party. Catholics have been a swing vote also for decades. Opinion polls show white Catholics evenly split between Obama and Republican John McCain.
John Kennedy became America's only Catholic president in 1961. If elected, Biden would be the first Catholic vice president. House Speaker Nancy Pelosi, who is next in line to the president after the vice president, is also a Catholic.
Faith features in US elections despite a formal separation of church and state in the workings of government.
Candidates are often asked about the role of faith in their lives and how it might shape their presidencies.
Biden's broad support for abortion rights could help with more liberal Catholics while his opposition to late-term abortions could attract those who are uncomfortable with the procedure but don't vote on it.
Opinion polls show that abortion, a traditionally divisive issue for Americans, is less of a factor for most people in this election than the economy, the Iraq war and health care, which top their priorities. …