It is a different story in western counties, where voters must choose between a Republican and Democratic ballot.
President Donald Trump slammed Republican Congressman Mark Sanford for not supporting his agenda, alluded to the lawmaker's extra martial affair and endorsed his primary opponent - all with less than three hours before polls close in SC.
He will face off against Arrington, a state representative, in Tuesday's GOP primary for South Carolina's 1st District.
As recently as the 1990s, Democrats and Republicans were fairly even divided in Nevada, a state that had supported Republican presidents including Ronald Reagan, George H.W. Bush and his son, George W. Bush. As voters head into the polls, six Democrats are still competing.
Nevada is crucial to the Democrats' chances of winning back the Senate. Dean Heller. And in North Dakota, Republicans will pick Rep. Kevin Cramer to challenge Democratic Sen. Formerly a safe Republican district, the 7th made national news in 2014 when Brat ousted Rep. Eric Cantor, then the house majority leader, in a shocking primary.
Traditionally, only 15 percent of registered voters turn out for primaries. He condemned Trump's travel ban and support for increased tariffs. Democrats need a net gain of 23 seats after the midterm elections to take control of the U.S. House.
"Any of those Democrats will be a threat to Comstock in November", said University of Virginia political scientist Larry Sabato, noting her district has shifted slightly to the left and that Democrats are fired up in opposition to Trump.
He's facing off against Mary Jones in Tuesday's primary.
There are three candidates, including Sanford, in the Republican primary.
Locally in the 2nd Congressional District, incumbent Republican Rep. Scott Taylor has some competition.
"This is about a president who presents a grave danger to our democracy", he told ABC News. Still, she'll face a hard general election against Rep. Kevin Cramer, R-N.D., the likely GOP Senate nominee.
The surprise of the day came just after 4 p.m. when Trump's official Twitter account posted a message endorsing state Rep. Kati e Arrington over Sanford. Arrington has sought to use Sanford's frequent criticism of Trump as a wedge with GOP primary voters.
Another Tuesday, another wave of primaries.
One of the hottest races is for the Republican nomination to challenge Democratic U.S. Senator Tim Kaine.
The fact that Heitkamp is running unopposed this year speaks to her strong standing with the state's Democratic Party.
Dave Moore, an American party candidate, will appear on November's general election ballot as well.
In the 6th District, voters are deciding the Democratic nominee to face off with GOP nominee Ben Cline.