Sen. Jim DeMint (R-S.C.), considered a hero in the tea party movement and a thorn in the side of Senate leaders, endorsed Delaware Senate candidate Christine O’Donnell and New Hampshire Senate candidate Ovide Lamontagne in posts on his Twitter account Friday night.

Lamontagne and O’Donnell are running in competitive primaries Tuesday, and neither Republican is the desired nominee of national party leaders. The endorsements serve as the latest examples of DeMint bucking the GOP establishment and Senate leaders in competitive primaries across the country this cycle.

“I’m proud to endorse Christine O’Donnell for US Senate in Delaware,” DeMint tweeted. “She will stand strong for the principles of freedom.”

Of Lamontagne, he wrote, “He’s a proven conservative.”

O’Donnell and Lamontagne are underdogs attempting to pull off big upsets Tuesday, and they have been gaining momentum over the past week by collecting some big endorsements. O’Donnell got the support of former Alaska Gov. Sarah Palin earlier in the week, and on Friday, the National Rifle Association announced it was backing her in the primary. Lamontagne was endorsed by New Hampshire’s biggest newspaper, the Union Leader.

In the Delaware race, O’Donnell is running against longtime Rep. Mike Castle, a popular moderate Republican who has led in the polls and has wide establishment support. The Tea Party Express is spending heavily in the Delaware race, sensing it is the next best opportunity to upend an establishment candidate after helping attorney Joe Miller upset Alaska Sen. Lisa Murkowski in the GOP primary.

If he is the nominee, Castle is viewed as the favorite to win the Senate seat formerly held by Vice President Joseph Biden in November. However, if Republicans nominate O’Donnell, Democratic nominee Chris Coons would be considered the heavy frontrunner to hold the seat for his party.

Lamontagne is running against a number of other Republicans in New Hampshire, including state Attorney General Kelly Ayotte, who was recruited into the race by national party operatives. Polls have shown him gaining some momentum, and he is also viewed as a tea party favorite in the race. The winner Tuesday will face Democratic Rep. Paul Hodes in November in a contest that should be competitive.

DeMint has been on the other side of Senate leaders in issuing endorsements in a number of Republican primaries in 2010. He supported Ken Buck in Colorado, Rand Paul in Kentucky and Marco Rubio in Florida, all of whom were eventually nominated.