The House passed a $600 million border security bill Tuesday by voice vote, kicking off a highly unusual, mid-August special session that Democrats have geared toward boosting their vulnerable Members’ re-election prospects.

Senate Republicans unexpectedly allowed the bill to pass in that chamber Thursday by unanimous consent.

The measure was a last-minute addition to the House’s agenda after Speaker Nancy Pelosi (D-Calif.) summoned House Members back to Washington for a one-day session to send an unrelated $26 billion state aid package to President Barack Obama for his signature.

However, the border bill must go back to the Senate, which already has left town for its five-week summer recess, because the House altered the measure slightly before passing it.

Still, the House action allows vulnerable Democrats, particularly those from border states, to burnish their enforcement credentials at a time when public frustration at Congressional inaction on the immigration issue is mounting.

Rep. Hal Rogers (R-Ky.), the ranking member of the House Appropriations Subcommittee on Homeland Security, accused Democrats of “playing politics with national security.”

Although no Republicans called for a recorded vote, GOP lawmakers chided Democrats for failing to pass the homeland security appropriations bill, which funds immigration enforcement programs.

Judiciary ranking member Lamar Smith said House passage of the bill “represents an opportunity lost.”

“If the Democrats were serious about immigration enforcement, they’d include more funds for interior enforcement,” the Texas Republican said.

Arizona Rep. Gabrielle Giffords urged Senators to also interrupt their summer recess to send the measure to Obama’s desk.

“This should be a bipartisan issue, and I urge the Senate to return immediately to pass this bill,” the Democratic lawmaker said.

Later Tuesday, the House is expected to vote on the $26 billion package of aid to states designed to keep teachers and other government workers from losing their jobs.