Julián Castro, the former secretary of Housing and Urban Development during the Obama administration, is campaigning for the Democratic frontrunner in a special election in Georgia.

Castro is backing Jon Ossoff, the leading Democratic candidate to fill the seat of former Rep. Tom Price (R-Ga.), who became the Trump administration's secretary of Health and Human Services.

Castro, who also served as a mayor of San Antonio, Texas, before joining the Obama administration, greeted volunteers at Ossoff's office on Saturday as Democrats try to flip the red seat.

Former Secretary @JulianCastro pumps up a room of volunteers to Get Out the Vote for @ossoff and #FlipThe6th pic.twitter.com/wJsu0RSEgd — Diana Bonete (@DianaBonete) April 15, 2017

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Several Republicans are also competing for the open Georgia seat, including state Sen. Judson Hill, former Georgia Secretary of State Karen Handel, former Johns Creek city councilman Bob Gray and others.

According to local NBC affiliate Alive 11, former House Speaker Newt Gingrich (R-Ga.) is voicing his support for Hill in a recorded robocall on Saturday.

Another notable GOP endorsement came from Karen Giorno, a former state strategist for President Trump's 2016 campaign, who thew her support behind Gray.

Democrats are scrambling to flip the red seat in Tuesday's special election and turn a victory into a referendum on Trump.