All the "pro-gun" Democrats on US Senate Homeland Security Committee vote against amendment to end gun-free zones in US Post Offices
Mark Prior: No
Jon Tester: No
Mark Begich: No
Heidi Heitkamp: No
The vote was 9 against - 6 in favor. If just two of these "pro-gun" Democrats had supported the amendment, it would have passed.
Over all, all the Republicans who voted supported ending gun-free zones at post offices and all the Democrats voted to keep them. Even John McCain voted to end these gun-free zones. The one exception to this pattern was Porter from Ohio (R) who was absent.
Democrat Mark Begich tried to give Democrats cover for their opposition by having a vote on allowing permit holders to lock their guns in their cars while they are in Post Office parking lots, though Begich himself said that his amendment was merely codifying recent court rulings on this point and not changing anything.
UPDATE: An additional discussion is provided here.