Democratic challengers in some of the closest Senate races tapped a fundraising vein in the last three months that exhibited momentum for their campaigns and supplied them with money to compete against better financed opponents.

Democrat Bob Casey in Pennsylvania reported raising $3.94 million in his campaign to unseat Republican Sen. Rick Santorum. The Santorum camp reported having $3.6 million in the bank at the end of September; Casey reported $3.7 million in hand.

In Virginia, Democrat Jim Webb reported raising $3.3 million from July to the end of September, with $2.73 million in the bank. Webb is challenging Republican Sen. George Allen, once considered a potential presidential candidate in 2008. Allen reported raising $1.94 million in the last quarter, but still had $5.57 million in hand, according to reports filed with the Federal Election Commission.

The numbers serve as one barometer of candidate strength. Incumbents typically have larger amounts of cash in hand because their fundraising machinery has been operating far longer.

Candidates in most close House and Senate races are also receiving help from the national parties, who help supplement campaign spending with party ad buys, polls and direct mail. As a result, the candidates' campaign finance reports tell only a part of the story.

With just three weeks left before Election Day, the parties are regularly reassessing their candidate's strengths and increasing or decreasing their advertising presence accordingly.

For instance, Missouri Democrat Claire McCaskill outraised incumbent Republican Sen. Jim Talent by about $1 million. Talent reported $4.2 million still in the bank, while McCaskill reported a mere $246,000 cash on hand.

But the national political parties have descended on the state, devoting millions on attack ads against each candidate — a spending commitment that shows no sign of abating. The Republican National Committee on Monday reported spending $701,718 in an anti-McCaskill ad buy in the state.

In Ohio, Rep. Sherrod Brown, the Democrat seeking to unseat two-term Republican Sen. Mike DeWine, collected $2.9 million in contributions, ahead of DeWine, whose camp said he raised $2.7 million. Brown had $1.2 million in hand, compared with $4.5 million for DeWine. Brown aides said the Democrat's numbers were lower because he had already paid for television ads that will run through the remainder of the campaign.

Some Republicans also showed fundraising strength. Republican Mike Bouchard in Michigan raised $2.15 million in his effort to unseat Democratic Sen. Debbie Stabenow, who raised $1.1 million. Stabenow held a similar 2-to-1 edge in available cash, however. She reported $1.25 million in the bank compared with $600,000 for Bouchard.

In New Jersey, Republican Tom Kean Jr. raised $1.76 million in his challenge to Democratic Sen. Robert Menendez, who raised $1.58 million during the same July-to-September period. Menendez had $5.5 million in hand and Kean had $3.17 million.

On Monday, the National Republican Senatorial Committee announced it was making its first move of the campaign into the expensive New Jersey media market, spending $500,000 to continue a Kean ad against Menendez in New York television stations.

Meanwhile in Connecticut, the parties are staying away, leaving Sen. Joe Lieberman, a Democrat running as an independent, to fend off a challenge from Democratic millionaire Ned Lamont. Lieberman reported raising $6.1 million between July 20 and Sept. 30, with $4.7 million on hand. Lamont reported raising $4.9 million, but $3.75 million of that was his own donation to the campaign. He reported $329,560 on hand.

In Minnesota's open Senate seat race, Democrat Amy Klobuchar raised $1.35 million in the five-week period ending Sept. 30, and had just under $2.25 million in the bank. Her Republican opponent, Rep. Mark Kennedy, raised $975,000 during the period and had just over $2.03 million in cash on hand. Democratic incumbent Sen. Mark Dayton is not seeking re-election.

In the open Tennessee Senate race, Republican Senate candidate Bob Corker, in a tight contest with Democrat Harold Ford, reported raising $3.9 million but had only $497,584 in the bank at the end of the month. Ford's figures were not immediately available.