DES MOINES, Iowa—Despite John McCain’s prediction of an upset, Barack Obama reached for a landslide Friday, invading his rival’s home state with TV ads and building a lead in early voting in key battlegrounds as the presidential race headed into a hectic final weekend.

McCain charged that Obama, bidding to become the first black president, “began his campaign in the liberal left lane of politics and has never left it. He’s more liberal than a senator who calls himself a socialist,” he added in Hanoverton, Ohio, a reference to Sen. Bernie Sanders, an independent from Vermont.

Yet with the economy almost certainly in a recession and the country clamoring for change after eight years of Republican rule, even some of McCain’s allies conceded the obvious. California Gov. Arnold Schwarzenegger said it would take a “major struggle for him to win”—although he quickly added the Arizona senator had come back before when he had been counted out.

Privately, McCain’s aides said their man trailed Obama by 4 points nationwide in internal polling.

An Associated Press-Yahoo News poll of likely voters put the Democrat ahead, 51 to 43, with a margin of error of plus or minus 3 percentage points.

The same survey gave McCain reason to hope—one in seven voters, 14 percent of the total—said they were undecided or might yet change their minds.