Backers of both Barack Obama and Hillary Rodham Clinton turned out with passionate support for their candidate in last week's Texas primary. But once they got in the voting booth, they did something different. Obama supporters were more likely to vote in the presidential race and then skip the other contests than Clinton supporters, who tended to continue voting down the ballot, a Dallas Morning News analysis finds. 

More than 80 percent of Democratic voters in the Texas counties where Mrs. Clinton had her largest victory margins went on to vote in the U.S. Senate race, the leading statewide contest on the ballot after the presidential race. By contrast, only 71 percent of voters in Mr. Obama's strongest counties did.

In Dallas County, where Mr. Obama got nearly two-thirds of the vote, the falloff was nearly 30 percent.

The analysis has limitations: It's impossible to tell which voters skipped the down-ballot contest, though the counties in question leaned heavily to one candidate or the other. And it cannot take into account the decisions of individual voters, driven by many different factors.

But the numbers suggest that many Obama voters were drawn singularly to him and might not return in the fall if he's not the nominee – blunting the flood of new voters who Democrats hope will help revive the party in Texas and sweep it into the White House.

"We wouldn't get a lot of those young voters that came into the process exclusively for him," said Dallas lawyer Doug Haloftis, a Clinton supporter.

It's a crucial point as the campaigns continue their long and possibly bloody battle for weeks, and perhaps months more. As they argue over familiar themes of experience and judgment, Democratic voters might become increasingly concerned with a more fundamental issue: Who can beat Republican John McCain in November?

The Obama campaign has attracted new voters, including young people and black voters "who had, by a factor of 10, more information on the presidential race than anything down-ballot," said Democratic strategist and Obama volunteer Glenn Smith.

"To get these people to return to the polls in November, the odds are much better if Barack Obama is the nominee," he said.

Garry Mauro, coordinator of the Clinton campaign in Texas, said he views the bigger drop-off rate among Mr. Obama's voters as a measure that they were more "candidate-oriented" than "issue-oriented."

"There are hundreds of people supporting Obama who don't have a clue why, so they can't vote for anybody else who they don't know anything about," Mr. Mauro said.

By contrast, he said, Mrs. Clinton is driving large numbers of women to the polls in Texas, a constituency that could offset any erosion should Mr. Obama's newfound primary voters stay home in November.

"We will have more women voters to the polls if Hillary is the nominee," Mr. Mauro said. "There is a whole body of political science knowledge that when you have a disproportionate number of women vote, that's when Democrats win."

Drop-off rate shifts

Statewide, nearly a quarter of Democrats cast ballots for president but didn't go on to vote for Senate.

In 1996, the last presidential year that included a competitive Senate primary, the drop-off rate between the presidential and Senate races among Democrats was 3 percent. The drop-off was higher in 2000, but there was no serious Senate contest that year.

This year, state Rep. Rick Noriega of Houston fended off three challengers to win the Democratic nomination.

Mrs. Clinton won the popular vote in the Texas primary, but Mr. Obama has a good chance to take more convention delegates from the state, thanks to results from precinct conventions held after the primary. Those votes are still being tabulated, and the delegates won't be finally awarded until after regional and state conventions.

There are several types of convention delegates, the voters who will determine the party's nominee at its national convention in August in Denver. After contests in three-quarters of the states, Mr. Obama leads Mrs. Clinton in "pledged delegates" – those whose votes are apportioned by the results of voting in states. Both sides are now actively appealing to party leaders known as superdelegates, who appear likely to have the final say on the nomination.

The fear among Obama supporters is that although he leads in delegates, he will be denied the nomination as a result of a deal negotiated by party leaders. If Mr. Obama holds a lead as the result of popular voting after all the primaries, his voters might allege a conspiracy against him if he doesn't get the nomination.

Paul Quinn College president Michael Sorrell, who helped raise money for Mr. Obama, questioned whether, if the Illinois senator is denied the nomination, Mrs. Clinton can attract his voters.

"Conventional wisdom says those people stay at home, but once they are in, you have a chance to keep them," he said.

GOP crossover

A review of the Texas vote shows that among the 15 counties Mr. Obama won with his biggest margins, the voter falloff between the president and Senate races ranged from 22 percent, in Harris County, to 38 percent, in Jefferson County.

The biggest falloff was in Republican-heavy Collin County, which Mr. Obama carried by 55 percent. Four in 10 Democratic voters who cast ballots in the presidential race didn't vote in the Senate race.

Republican strategist Royal Masset said the Collin County vote illustrates a big reason for the voter falloff – Republican crossover voters who wanted to influence the outcome.

Although some conservative talk show hosts had urged Republicans to cross over and vote for Mrs. Clinton in order to keep the contest going, there was little evidence that happened.

According to exit polls, only 9 percent of Democratic voters statewide identified themselves as Republicans, and they went for Mr. Obama, 53 percent to 46 percent.

Republican pollster Mike Baselice said a 9 percent to 15 percent crossover vote is typical in Texas, and early-voter analysis indicates many of the "new" voters had some history of voting Democratic in general elections and were only new to a primary.

On the Clinton side, her top 15 counties had a substantially lower voter falloff, from 11 percent in Webb County to 24 percent in Bowie County and 26 percent in Hidalgo County.

In South Texas, a Clinton stronghold, more people likely voted for both president and Senate because of the presence of a Senate candidate with a Hispanic surname.

Removing the South Texas counties from the equation did not significantly change the falloff numbers.

Mr. Haloftis of Dallas said he's confident that even if the party loses some Obama supporters, a Clinton candidacy would attract "strong support from Southern, blue-collar Democrats."

Jeff Strater, a client relations manager at a Dallas law firm who voted for Mrs. Clinton, was also optimistic that she would get strong Democratic support in November.

"Everyone wants to see an end to the current administration, regardless of who the current nominee is," he said. "We'll be focused on capturing the White House. And if it's Obama, I'm right there with them."