WASHINGTON -- The U.S. Chamber of Commerce jumped into two key Senate races this week -- another clear signal that the GOP establishment will try to stop tea party candidates from winning primaries as Republicans try to wrest control of the Senate from Democrats.

The chamber announced its support Thursday for Jack Kingston, the Republican congressman who is trying to emerge from a packed field of hard-right contenders in next month's Georgia primary for an open seat.

Republicans have openly worried that a tea party-aligned candidate could be too extreme for most of the voters in the state. Demographics there are tilting toward blue, giving the likely Democratic nominee, Michelle Nunn, a potential shot at the seat. Nunn posted a  strong first-quarter fundraising haul this week.

"Now more than ever we need conservative leaders with a demonstrated record," Rob Engstrom, the chamber's national political director, said in making the announcement.

Earlier this week, the chamber dived into the brutal GOP primary contest underway in Mississippi with an attack ad against "trial lawyer Chris McDaniel," the telegenic state senator who is trying to oust six-term Republican Sen. Thad Cochran.

Cochran also had an impressive fundraising tally, but McDaniel is being backed by outside conservative groups, including the Club for Growth, which have made his race a priority.

GOP leaders have decided against a laissez-faire approach to the primary battles, worried that the hard-right candidates could alienate voters in the November election -- as has happened before -- and dash hopes of picking up the six seats needed to flip control of the Senate.

lisa.mascaro@latimes.com

Twitter: @LisaMascaroinDC