Atlanta: Registered Democrats have out-voted Republicans by double-digit margins among ballots cast early in two bitter Senate races, but the national political climate favours Republicans to take control of the chamber with a week to go before midterm elections.

Republicans remain optimistic on their overall momentum to wrest the Senate majority from the Democrats, and are looking to expand their majority in the House of Representatives. Such an outcome would make it difficult for President Barack Obama to achieve many of his legislative goals during the remaining two years of his term, from raising the minimum wage to immigration reform legislation.

Democrats pressed on aggressively with efforts to motivate key voter groups, banking on high turnout among their core supporters to outweigh their main weakness heading into the November 4 elections: Obama’s low approval ratings.

Senate Democrats unleashed a late-campaign round of attack ads on Monday accusing Republicans in key races of harbouring plans to cut Social Security pension benefits and the Medicare health programme for the elderly. The ads appear aimed at older voters, who cast ballots in relatively large numbers in midterm elections and have tended to support Republicans in recent years.

And in Iowa, Vice President Joe Biden tried to fire up Democratic loyalists, saying that keeping Democratic control of the Senate would “break the back of the hard right” and ease the partisan divisions that have paralysed Congress

In Colorado, former President Bill Clinton urged Democrats to ignore polls showing their party is unlikely to retain control of the Senate. Clinton accused Republicans of trying to get people to “just vote your fears and your anger” as he campaigned for Senator Mark Udall who is locked in a tough re-election contest against Republican Rep. Cory Gardner.

The Republicans, who need six seats to take control of the Senate, have already used their majority in the House to thwart much of Obama’s agenda in the past four years, including stricter gun control laws.

State tallies of early voting put Democrats ahead of Republicans by nearly 18 percentage points among ballots cast early in Louisiana, where Sen. Mary Landrieu faces an uphill battle to keep her seat. However, that race is expected to head to a runoff since no candidate is likely to receive more than 50 per cent of the vote. In North Carolina, Democrats hold an early 17-point lead in early voting as another Democratic incumbent, Sen. Kay Hagan, fights for re-election.

Both margins are wider than in 2010, the last national midterms.

Yet in Iowa, the Republicans have narrowed the early voting gap with Democrats compared to the 2010 and 2012 elections, in what could be a good sign for their Senate nominee, state Senator Joni Ernst.

Biden said next week’s choice between Ernst and Democratic Rep. Bruce Braley is critical to the outcome of the bitter fight for control of the Senate. If Braley wins and the Democrats maintain their majority, Biden said, the Republicans in Congress will be open to compromising with Democrats on issues such as raising the federal minimum wage.

“If that happens and it will, what’s going to happen is it’s going to break the back of the hard right,” he said.

In Kansas, former Republican presidential nominee Mitt Romney campaigned with US Sen. Pat Roberts, in an effort to bolster the three-term incumbent’s message that Republican voters must re-elect him to thwart Obama’s agenda.

But independent candidate Greg Orman — who says he voted for Romney for president in 2012 after contributing to Obama’s 2008 campaign — said voters face a choice between maintaining gridlock and “actually solving problems.”

Roberts, 78, is in a tight race with Orman, a 45-year-old businessman and co-founder of a private equity firm. Republicans had counted on Roberts winning in his reliably Republican-leaning state, but an Orman victory would make it harder to regain a Senate majority.

Orman is running as a centrist, promising to caucus with whichever party wins a clear majority in the Senate or to play kingmaker if neither does. Roberts and his fellow Republicans portray Orman as a liberal Democrat in disguise as they try to keep disaffected Republicans and unaffiliated voters in Roberts’ camp. The Democratic candidate withdrew from the race to consolidate the anti-Roberts vote.

Obama is staying away from high-risk Senate races, where his poor approval ratings are a drag on Democratic candidates. Instead, the president is heading out this week for campaign appearances in six states where Democrats are in tight races for governorships but where his own approval ratings are solid.

A key question on early voting is whether either party is actually coaxing new voters to the polls. If they’re just cutting into their usual Election Day turnout, early voting isn’t as likely to alter historical trends, which favour Republicans as young and minority voters — who lean Democratic — are more likely stay home in elections when the presidency isn’t at stake.

Nationally, almost 8.6 million Americans have cast ballots so far in 27 states. In 2010, nearly 27 million people cast ballots, including absentee, before the November election date, accounting for about 30 per cent of the total electorate.