Software maker Blackbaud Inc. has once again extended its $275 million proposal to buy a Texas competitor to allow federal antitrust regulators more time to review the transaction, the Daniel-Island-based company announced in a filing with the Secutiries and Exchange Commission this week.

The new deadline to close the acquisition of Convio, which also makes fundraising software for nonprofits, is April 4, according to the Wednesday filing.

The date, which has been postponed three times now, can be extended through September. In January, when the acquisition was announced, Blackbaud CEO Marc Chardon predicted the deal would close in the first quarter of 2012, but that now appears unlikely.

More than half of Convio's almost 19 million shares have been tendered, exceeding the approval threshold. The last hurdle is the Department of Justice review.