The former Obama administration official and San Antonio mayor Julian Castro has issued a warning to Senator Ted Cruz in the wake of a shock Democrat win in Alabama.

Democrat Doug Jones claimed victory over Republican Roy Moore in Alabama on Tuesday, meaning the traditionally conservative stronghold has its first Democratic senator since 1992.

The result dealt a political blow to Donald Trump since it narrowed the slim Republican majority in the Senate to 51-49.

After the win, Mr Castro – who served as Secretary of Housing and Urban Development under Barack Obama – took to Twitter.

"Be afraid, Ted Cruz. Be very afraid," he said in a post.

Mr Cruz – who ran against Mr Trump for the Republican presidential nomination – is facing a challenge to his Senate seat in Texas in 2018 from Beto O'Rourke.

Mr O'Rourke, who has served in Congress since 2013, is expected to struggle against Mr Cruz, since no Democrat has won a Senate election in Texas since 1988.

But Mr Jones's upset win against Mr Moore, a former state judge who found himself at the centre of a sexual harassment scandal, has energised the Democratic Party.

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An attorney and former prosecutor, Mr Jones rallied voters on a message of moving past Mr Moore's controversies. He was buoyed by an influx of national Democratic cash and endorsements.