Republican nominee Mitt Romney out raised President Obama by $21 million in the first half of October, apparently gaining momentum from a strong debate performance and tightening polls.

"BOOM: In first half of October alone, @MittRomney effort raised $111.8 million," campaign spokeswoman Andrea Saul wrote in a tweet Thursday morning. The influx of $111.8 million from Oct. 1 to Oct. 17 left Romney and the Republican Party with nearly $170 million cash on hand, officials said. Obama and the Democrats raised $90.5 million during the same period; it did not immediately release a cash figure.

The haul coincided with Romney's strong performance in his Oct. 3 debate with President Obama in Denver and a subsequent tightening in public opinion polls. The Romney campaign, which relies more heavily on large-dollar donations than Obama, has kept up a furious pace of fundraising even in the final weeks of the campaign, including a series of events in solidly red states this week held by Rep. Paul Ryan (R-Wis.), Romney's vice presidential running mate.

Obama had edged out Romney in fundraising in recent months. Both sides have now eclipsed $1 billion each between the campaigns, political parties and super PACs.

The Romney campaign said it had about $169 million in cash on hand between its various committees, a stunning amount this late in the game. But the airwaves are already saturated in swing states and grassroots efforts take time, leaving open the question of how much help the cash will give him in the 12 days until the election.

The campaigns are due to file their last full pre-election disclosure reports with the Federal Election Commission by midnight Thursday.