WASHINGTON — Sen. Barack Obama has picked up substantial support in recent days from a crucial constituency — female voters — but the gains could be fleeting.

A Gallup poll taken last Thursday through Monday, as Sen. Hillary Clinton left the race, found Obama beating presumptive Republican nominee Sen. John McCain among women 51-38 percent, up from 48-43 percent a week earlier.

More important, he had opened up a 47-41 percent edge among women over 50, the core of Clinton's support. The previous week, McCain had been ahead among them 46-43 percent. Among all voters, Obama led 48-42 percent.

"In part it's a bump" because the Illinois senator became the presumptive Democratic nominee, said John Pitney, a professor of American politics at Claremont McKenna College in California.

In other news Wednesday from the Democratic candidate's campaign, the former CEO leading Obama's search for a vice presidential candidate resigned abruptly after questions arose about his home mortgage deals.

Jim Johnson's resignation came a day after Obama defended Johnson and dismissed the Republican criticism of him.

Obama accepted Johnson's resignation on Wednesday, leaving the search at least temporarily in the hands of two other prominent Democratic attorneys who also had been scouting candidates — former Deputy Attorney General Eric Holder and Caroline Kennedy.

"Jim did not want to distract in any way from the very important task of gathering information about my vice presidential nominee, so he has made a decision to step aside that I accept," Obama said.

Johnson, the former chairman of mortgage lender Fannie Mae, came under fire from GOP candidate John McCain and the Republican Party after The Wall Street Journal reported last Saturday that Johnson got mortgages with help from the CEO of Countrywide Financial Corp.

McCain had accused Obama of hypocrisy for speaking out against Countrywide's role in the subprime mortgage crisis.

Obama's campaign declined to say whether he would replace Johnson, who also led the vice presidential search effort for Democratic nominee John Kerry in 2004 and Walter Mondale in 1984. Obama said he was grateful for Johnson's service.