SELMA, Ala. — Evoking the passions and rivalries of the civil rights era, Sens. Barack Obama, D-Ill., and Hillary Rodham Clinton, D-N.Y., made deeply personal appeals to voters in the sanctuaries of black churches here on Sunday, and then joined former President Bill Clinton for a march across a bridge where white police beat black Alabamians nearly 42 years ago.

It was an extraordinary sight: The Clintons and Obama, competitors for the Democratic presidential nomination, locking arms — with two black congressman in between them — and walking down Martin Luther King Jr. Street to commemorate the footsteps of black demonstrators who were met with violence as they tried to march to Montgomery to demand civil rights in 1965.

The visit to Selma, a historically rich, economically struggling city, became a proxy battle for black support between Hillary Clinton and Obama, whose historic candidacy represents a threat to Clinton's traditional base. That competitive dynamic intensified Sunday with the debut of Bill Clinton on the campaign trail, six weeks into his wife's bid, and among a bloc of voters who are at once devoted to the former president and torn between his wife and Obama.

It was the first side-by-side appearance of Obama and Hillary Clinton in their 2008 presidential campaign, and the political theater of the two campaigns overlapped repeatedly, but with a polite tone that contrasted with their political skirmishing of recent weeks. Obama and Hillary Clinton spoke at services on the same street, three blocks apart, and the lines of worshippers were so long that they nearly intermingled. Both candidates paid homage to the same civil rights leaders, and both concluded the services by locking arms with worshippers and swaying to "We Shall Overcome."

Bill Clinton, arguably the most cadence-blessed speaker of the three, half-joked Sunday afternoon that he had been bested by the other two. "All the good speaking has been done by Hillary and Senator Obama already — I'm just sort of bringing up the rear," he said.