The run-up to fall's mid-term elections in the United States heated up on Tuesday, with party primaries in six states that could eventually decide who controls the U.S. Senate.

Republican primary struggles between establishment-backed conservatives and tea party-favoured rivals were a dominant feature in several states, as they had been earlier in North Carolina and will be later in Mississippi, Kansas and Alaska.

Republicans must gain an additional six seats to win a Senate majority, and party leaders have made it a priority to avoid the presence of candidates on the ballot this fall who are seen as too conservative or unsteady — or both — to prevail in winnable races.

In Kentucky, Senate Republican leader Mitch McConnell looked to vanquish tea party challenger Matt Bevin, who was backed by tea party groups in the state where they made their mark four years ago by sweeping GOP Senator Rand Paul into office.

In Arkansas, Democratic Sen. Mark Pryor and his Republican challenger, Rep. Tom Cotton, ran unopposed for their party nominations, pointing toward their highly anticipated battle in November as the GOP labours to gain a majority.

Pennsylvania Democrats eager to gain control of their statehouse sorted through four contenders vying to challenge Republican Gov. Tom Corbett, who faces an uphill battle for re-election in November.

Idaho and Oregon also held primaries on the busiest night of the year so far on the political calendar.

Out-manoeuvred in 2010 when his preferred contender was defeated, McConnell responded this time by hiring a top aide to Paul as his own campaign manager. For his part, Bevin stumbled through a campaign that included an appearance at a rally of cock-fighting supporters.

Expensive, competitive races

Alison Lundergan Grimes, touted by Democratic Party leaders in Washington, faced three rivals in her bid to oppose McConnell in the fall in a race that is likely to be among the most expensive and competitive in the country.

The Arkansas race was in that category, too, as was Georgia's.

In Georgia, Michelle Nunn, another prized Democratic recruit, sought the nomination to the seat her father, Sam Nunn, held for four terms.

Businessman David Perdue, Rep. Jack Kingston and former secretary of state Karen Handel were among seven trying for the Republican nomination in a struggle likely headed for a two-way runoff on July 22. Reps. Phil Gingrey and Paul Broun also were on the ballot, and the presence of three incumbent lawmakers on the Senate ballot assured a large turnover in the state's House delegation come January.

The Senate race was fiercely expensive — $10 million had been spent on television commercials through the end of last week — and highlighted the divisions within the Republican party. Perdue relied on his background as a businessman, while Broun and Gingrey ran farther to the right. Handel sought to capitalize on the backing of former Republican vice presidential candidate Sarah Palin, and Kingston had the support of the U.S. Chamber of Commerce.

In Oregon, Republicans picking a nominee to oppose Sen. Jeff Merkley chose between state Rep. Jason Conger and Monica Wehby, a physician.