Voters went to the polls for primary elections in six US states on Tuesday, with Republican leaders hoping to set the stage for regaining full control of Congress in November.

Republicans control the House of Representatives and are determined to wrest the Senate from President Barack Obama's Democrats in the mid-term elections later this year.

All eyes will be on Kentucky, a key battleground between traditional Republicans and members of the party's more conservative, populist "Tea Party" wing.

Incumbent Senate Minority Leader Mitch McConnell, 72, is facing a primary challenger endorsed by several Tea Party groups, Matt Bevin, who wants to oust McConnell three decades after his first win.

The anti-establishment fervor sweeping much of the country seems not to have taken as strong a hold in Kentucky, with polls showing McConnell likely to win.

If he is reelected in November against his Democratic rival, and if Republicans gain a net six seats in the 100-seat chamber to regain control, McConnell would become head of the Senate majority and be positioned to block Obama's legislative efforts in his last two years in the White House.

- More primaries June 3 -

Arkansas, Georgia, Idaho, Oregon and Pennsylvania also hold primaries Tuesday. On June 3, another eight states follow suit.

The mid-term elections, so called since they come at the mid-point of the presidential term, are scheduled for November 4.

The entire House, currently held by Republicans, is up for grabs. Analysts expect the party will retain control; the question is whether Democrats can cut into the Republicans' majority.

The Democrat-led Senate could swing the Republicans' way, polls show, so the Senate races are among the most fiercely fought.

More than a third of the seats (36 of 100) are in play for six-year terms, and at least seven Democrat-held seats are seen as at risk.

With control of the chamber in play, candidates and supporters are spending tens of millions of dollars on each Senate race amid an epic battle over who controls the legislative agenda in Washington.

Republican strategists want to line up fairly mainstream candidates who are not gaffe-prone.

In 2010 and 2012, Tea Party-backed candidates won primaries in Delaware, Indiana and Missouri but fizzled in the election.

Faced with the Tea Party surge, some mainstream candidates have adopted a more conservative discourse: speaking out against big government, stressing the need to cut spending or embracing more conservative social positions on abortion or gay marriage.

For example, North Carolina state legislator Thom Tillis, who is running for the Senate, defines himself as "very conservative" and backed a state constitutional ban on same-sex marriage.

"It may be that the establishment has moved to the right sufficiently that it's hard to tell the difference between the Tea Party types and non-Tea Party establishment types," said John Aldrich, a political science professor at Duke University.

Tea Party-backed candidates are running in nearly all of Tuesday's primaries.

One of the races that have lured attention is in mostly rural Iowa on June 3.

Joni Ernst, a National Guard officer sent to Kuwait in 2003, made headlines when she said in a campaign ad: "I grew up castrating hogs on an Iowa farm, so when I get to Washington, I'll know how to cut pork."

Pork is US political slang for politicians' pet projects, for which they seek often hefty spending.