Former Democratic presidential candidate Julian Castro campaigns with Jon Ossoff in Georgia Senate runoff

Georgia Senate candidate Jon Ossoff on Monday campaigned with former Democratic presidential candidate Julián Castro, adding yet another name onto the long list of boldface politicians who have stumped for both sides in the critical runoffs. 

Castro, the former secretary of Housing and Urban Development (HUD), appeared with Ossoff at a Latino voter registration rally Monday and is set to appear at multiple other events Monday and Tuesday. Castro's former boss, former President Barack Obama, as well as President Donald Trump, Vice President Mike Pence and several senators have actively stumped for either the Republicans or Democrats in the state. 

President-elect Joe Biden has urged his Twitter followers to register to vote in Georgia. And Ron Klain, whom Biden selected as his White House chief of staff in his incoming administration, said it's likely Biden will campaign in Georgia before the Jan. 5 elections, which will determine which party controls the U.S. Senate. 

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"If we don't get out and vote, you know what's going to happen: All of these great ideas, all of these investments that could make people's dreams a reality — they’re going to get sidelined," Castro said at the Monday rally, according to the Atlanta Journal-Constitution. 

Ossoff campaigning with Castro also continues a trend of both parties aiming to put away their intra-party differences for the sake of winning the critical Georgia elections. 

Ossoff supports a public health insurance option added to the Affordable Care Act, which is the health care plan Biden supports. But he's nevertheless accepted endorsements from Castro and Sen. Bernie Sanders, I-Vt., who were some of the top proponents of a Medicare-for-all single-payer system during the Democratic presidential primary. 

On the GOP side, Maryland Gov. Larry Hogan, one of the top antagonists of the president from inside his own party, has enthusiastically endorsed Sen. David Perdue, R-Ga., who is Ossoff's opponent, and Sen. Kelly Loeffler, R-Ga., who faces Democrat Raphael Warnock on Jan. 5.

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And so has Donald Trump Jr., the president's son, who is appearing in campaign ads aimed at juicing turnout among Trump voters for the GOP senators. 

Republicans currently have locked down 50 seats in the Senate for the next term, and therefore need to win only one of the Georgia elections to ensure a majority and a significant say over the Biden agenda. But if Democrats sweep the Georgia runoffs they will bring the body to an effective 50-50 tie, allowing Vice President-elect Kamala Harris to break ties on party-line votes. 

Issues likely to be on the next Congress' agenda range from Biden's executive and judicial appointments to his public health insurance option to a rollback of the 2017 tax cuts. 

Fox News' Paul Steinhauser contributed to this report.