WASHINGTON — President Barack Obama returned to the White House on Thursday from a vacation shortened by government gridlock while Democrats and Republicans snarled across a partisan divide and showed no sign of compromise to avoid year-end tax increases and spending cuts. Adding to the woes confronting the middle class was a pending spike of $2-per-gallon or more in milk prices if lawmakers failed to pass farm legislation by year’s end. White House aides disputed reports that Obama was sending lawmakers a scaled-down plan to avoid the “fiscal cliff” of tax increases and spending cuts. They gave no indication he …