By Greg Giroux, CQ Staff

One reason why Democrats are expected to make gains in the U.S. House in the November elections is they are fielding a number of well-financed challengers to Republican incumbents.

Democrats comprise nine of the 10 most cash-rich challengers to incumbents of the opposite party, according to a CQ Politics analysis of campaign finance reports that were recently filed with the Federal Election Commission. The large cash-on-hand totals posted by these Democrats are one sign party officials are hopeful of a number of seat takeovers in November.

The challenger candidate with the most cash-on-hand is Connecticut Democrat Jim Himes, a former investment banker who has $1.4 million on hand as July began to oppose Republican Rep. Christopher Shays . Himes is followed by Sandy Treadwell, a former New York Republican chairman who has $1.3 million in his account to challenge Democratic Rep. Kirsten Gillibrand .

Treadwell is the only Republican on this Top 10 list. If the list of opposite-party challengers with the most cash-on-hand is extended to 20 House candidates from 10, the partisan ratio becomes less lopsided, with 13 Democrats and 7 Republicans.

In some cases the Democratic challenger has even more cash-on-hand than the Republican incumbent. Darcy Burner, a Democrat from Washington state, reported having more to spend than Republican Rep. Dave Reichert , who narrowly beat Burner in 2006.

Some of these Democratic challengers may also benefit from added assistance from the Democratic Congressional Campaign Committee, the campaign arm of House Democrats that has tens of millions of dollars more than its partisan counterpart, the National Republican Congressional Committee, to spend on television ads and other campaign communications.

The list below shows those opposite-party challengers who had the most remaining campaign cash on hand. In most cases, the figures are current to June 30, the end of this year’s second quarter. Candidates in Oklahoma, Missouri, Michigan and Kansas, where there are primary elections in the next seven days, were exempt from filing a second-quarter report. Candidates in those states instead filed “pre-primary” reports that detailed campaign finance activity through July 16.

1. Jim Himes, Democrat, Connecticut’s 4th ($1.4 million). Himes is challenging 10-term Rep. Shays in a district that includes Bridgeport, Stamford and other southwestern Connecticut communities near New York City. Long the preferred candidate of Democratic officials in Washington and Connecticut, Himes faces a Aug. 12 primary with little-known Democrat Lee Whitnum that Himes should win easily and that shouldn’t require him to draw down his treasury too much. Shays has $1.7 million left in his campaign account. CQ Politics Race Rating: No Clear Favorite.

2. Sandy Treadwell, Republican, New York’s 20th ($1.3 million). Treadwell, a former New York Secretary of State, is the leading Republican candidate against first-term Rep. Gillibrand, whose district along the eastern border of New York stretches from Lake Placid in the north to near Poughkeepsie in the south. Both Treadwell and Gillibrand have been among their respective party’s best-funded candidates throughout the two-year campaign cycle. Gillibrand, who has $2.8 million left to spend, is unopposed in the Sept. 9 primary election, while Treadwell is favored against two lesser-known Republicans. CQ Politics Race Rating: Leans Democratic.

3. Darcy Burner, Democrat, Washington’s 8th ($1.2 million). Burner, who was formerly employed by Microsoft, is taking on two-term Rep. Reichert ($916,000) in a suburban Seattle district in which she came within three percentage points of unseating the congressman in 2006. Burner’s challenge is one reason why Reichert is among the most vulnerable Republican incumbents; so too is the likelihood that his district will back Barack Obama over John McCain for president. CQ Politics Race Rating: No Clear Favorite.

4. Dan Seals, Democrat, Illinois’ 10th ($1.18 million). Seals, a business consultant, is waging a rematch campaign against four-term Republican Rep. Mark Steven Kirk , who won by a margin of 53 percent to 47 percent that was closer than in his previous two re-election campaigns. Seals’ respectable showing spurred him to try again — and begin his rematch campaign at an earlier point in the campaign cycle. That helps explain why Seals is among the best-funded challenger candidates. But Kirk has been ramping up his fundraising, tallying $3.8 million in receipts since the beginning of 2007 and posting $2.9 million cash-on-hand as July began. The latter figure is the second-highest among House Republicans. CQ Politics Race Rating: Leans Republican.

5. Gary Peters, Democrat, Michigan’s 9th ($1.1 million). Peters, a former state senator, is taking on eight-term Republican Rep. Joe Knollenberg ($1.9 million) in a district that takes in eastern Oakland County suburbs near Detroit, like Farmington Hills, Troy and Rochester Hills. Peters is substantially better-funded than 2006 nominee Nancy Skinner, who raised and spent a little more than $400,000 in taking 46 percent of the vote against Knollenberg. Peters and Knollenberg, whose cash-on-hand totals are current as of July 16, are unopposed in the Aug. 5 primary. CQ Politics Race Rating: Leans Republican.

6. Raul Martinez, Democrat, Florida’s 21st ($1.08 million). Martinez is a former mayor of Hialeah, a south Florida city of 226,000 residents, most of whom live in the 21st. The Republican incumbent is Lincoln Diaz-Balart ($1.8 million), who has run without Democratic opposition in six of his eight elections. One of the exceptions was in 2006, when Diaz-Balart took 59.5 percent of the vote against a little-known Democrat. CQ Politics Race Rating: Leans Republican.