WASHINGTON — As Congress returned from a seven-week recess on Tuesday, Senate Democrats again stymied a $1.1 billion plan to fight the Zika virus, demanding that Republicans drop an effort to block Planned Parenthood from receiving money to combat the mosquito-borne disease.

Democrats, who had essentially blocked the same legislation in late June, had enough votes Tuesday to prevent Congress from moving emergency funding public health experts say is desperately needed as they prepare for the possibility that Zika will spread to other states along the gulf coast. The vote was 52 to 46, and Republicans needed 60 votes to advance the bill.

Senator Mitch McConnell of Kentucky, the Republican majority leader, said that although efforts to fight the virus had produced encouraging results, the problem was far from over.

“It’s hard to explain why, despite their own calls for funding, Democrats would block plans to keep women and babies safe from Zika,” Mr. McConnell said before the vote.