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

Bob Casey, a Democrat of Pennsylvania, reported raising $3.94 million in his campaign to unseat Senator Santorum, a Republican. 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. Mr. Webb is challenging Senator Allen, a Republican, once considered a potential presidential candidate in 2008. Mr. 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.

Missouri Democrat Claire McCaskill outraised incumbent Senator Talent, a Republican, by about $1 million. Talent reported $4.2 million still in the bank, while Ms. 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.