WASHINGTON - Early votes soared past 15 million across 31 states on Friday, an outpouring that is giving hopeful Republicans as well as nervous Democrats cause for optimism heading into the final weekend of a campaign with control of the Senate, the U.S. House and 36 governorships at stake.

Republicans pointed to a strong early-vote performance in Iowa as evidence that Joni Ernst was a step ahead in her bid to capture a Senate seat for the GOP. "I feel real good about it," said Gov. Terry Branstad, campaigning with the party's Senate hopeful as he sought a new term for himself, as well.

But in Georgia, Democrats said a strong early turnout by African-Americans in the counties around Atlanta was a good sign for Michelle Nunn, running for a seat long out of the party's reach.

As candidates headed into a final weekend of campaigning, Democratic hopes of holding a Senate seat in Arkansas appeared to be fading, and Republicans already appeared assured of gains in West Virginia, South Dakota and Montana. They need to gain six to come away with the election's biggest prize - control of the Senate during President Barack Obama's final two years in office.

Strategists in both parties agreed privately that races in Louisiana and Georgia were probably headed for runoffs, and several Republicans expressed concern about Kansas, where polls showed Sen. Pat Roberts was in a tough race to keep a seat held by Republicans for decades.

Democratic incumbents in Alaska, Colorado and New Hampshire also faced difficult tests, as did Senate Republican Leader Mitch McConnell in Kentucky. In Idaho, first-time Democratic candidate Nels Mitchell is running against Republican U.S. Sen. Jim Risch, who is seeking a second six-year term.

Republicans are all but certain to hold their majority in the House, and Democrats are working to minimize their losses in the campaign's final days by rushing help to candidates once viewed as safe.

The early vote turnout varied from state to state, exceeding 2010 levels in Louisiana, Wisconsin, Maryland, Florida, Iowa and Georgia. Most of those states are holding intensely competitive races for the Senate or governor or both.

In Louisiana, where Democratic Sen. Mary Landrieu is in a difficult three-way fight for re-election, the early vote was 80 percent higher than in the 2010 midterm elections.

In Iowa, figures provided by state election officials showed Democrats had cast about 41 percent of the early vote, Republicans about 39 percent and independents nearly 20 percent. Four years ago, Democrats held a double-digit advantage.

In Georgia, though, official figures showed an increase in heavily African-American counties accounted for all of the gain in early votes over 2010. Democrats have worked for months to bring so-called drop-off voters into the electorate, those who usually skip midterm contests.

In addition to Nunn's race against Republican businessman David Perdue, Jason Carter is challenging Republican Gov. Nathan Deal in the state with the highest unemployment in the country.

Wisconsin voters exceeded their 2010 early vote totals by 10 percent. Democrat Mary Burke is challenging Republican Gov. Scott Walker in one of the country's most closely watched elections. No party breakdown was available.

The early vote in Kansas was just shy of the 2010 totals, about 153,000 overall. Nearly 55 percent were cast by Republicans, but GOP officials privately expressed nervousness.

Roberts isn't the only Republican with a tough fight. Gov. Sam Brownback, whose agenda has been so conservative that it sparked a rebellion within his own party, is running against Democratic challenger Paul Davis.