WASHINGTON -- Voters were picking Senate candidates Tuesday in states seen as pivotal to Republican hopes for capturing the U.S. Senate in November.

Six states were holding primaries Tuesday, the busiest day so far this year on the U.S. political calendar. The most closely watched was in Georgia, where seven Republicans were engaged in an expensive fight for their party's nomination. The southern state is one of the few where Democrats have hopes of winning a seat now held by Republicans.

Republicans have many more prospects for picking off Democratic seats, including in Arkansas, a conservative southern state where incumbent Mark Pryor faces a tough re-election challenge from Republican congressman Tom Cotton. Both were running unopposed for their parties' nominations Tuesday.

If Republicans capture the Senate, President Barack Obama likely would have to deal with a Congress entirely in opposition hands for the final two years in office. Republicans are expected to keep control of the House of Representatives.

Republicans must gain six seats to take control of the Senate. About a third of the 100 seats will be on the ballot. Democrats have been hurt by the retirement of some of their longtime senators in states that lean Republican. Also, some Democratic senators swept into office when Obama was first elected in 2008 are now vulnerable as they seek re-election.

Democrats are contending with Obama's low popularity and a history of a president's party generally losing seats in midterm elections. Republicans are making Obama and his troubled health care overhaul the focus of their campaigns.

The primaries are critical for Republicans, who saw an opportunity to win the Senate slip away in 2010 after party voters repeatedly picked candidates backed by the small-government tea party movement. Many were too conservative or unsteady to win in the general election.

This time, Republican Party leaders are determined to fend off tea party challenges. In one of the most high-profile races, the top Senate Republican, Mitch McConnell, defeated a tea party challenger in Kentucky. The tea party made its mark in that state four years ago by sweeping Rand Paul into the Senate over an establlishment candidate favoured by McConnell.

Out-manoeuvred then, McConnell responded this time by hiring a top aide to Paul as his own campaign manager. Meanwhile his rival, businessman Matt Bevin, stumbled through a campaign that included an appearance at a rally of cock-fighting supporters.

Alison Lundergan Grimes, a top state official, defeated three rivals in her bid to oppose McConnell in a southern state in which Democrats have a shot at unseating a Republican U.S. senator.

In another southern state, Georgia, the Democratic candidate, Michelle Nunn, is from one of the state's most famous political families. She's vying for the seat her father, Sam Nunn, held four terms.

Businessman David Perdue, Rep. Jack Kingston and former Secretary of State Karen Handel were among those seeking 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. The incumbent, Saxby Chambliss, is retiring.

The race was fiercely expensive -- $10 million had been spent on television commercials through the end of last week -- and highlighted the divisions within the party. Perdue relied on his background as a businessman, while Broun and Gingrey ran further 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 congressman Jason Conger and Monica Wehby, a physician.

There were also gubernatorial primaries in Pennsylvania, Georgia, Arkansas, Oregon and Idaho.

A few Republican members of the House of Representatives faced primary foes, notably Mike Simpson of Idaho. Challenger Bryan Smith said the incumbent wasn't conservative enough, but establishment groups have rallied behind Simpson.